Inventory, Net [Abstract] Materials, Supplies and Fuel Schedule of Inventory, Current [Table Text Block] Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Loss from discontinued operations, net of tax Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Income from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Adjustments to reconcile net income to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Depreciation, depletion and amortization Depreciation, Depletion and Amortization Deferred financing cost amortization Amortization of Debt Issuance Costs and Discounts Impairment of investment Other Asset Impairment Charges Stock compensation Share-based Compensation Deferred income taxes Deferred Income Tax Expense (Benefit) Employee benefit plans Pension and Other Postretirement Benefits Cost (Reversal of Cost) Other adjustments, net Other Noncash Income (Expense) Changes in certain operating assets and liabilities: Increase (Decrease) in Operating Capital [Abstract] Materials, supplies and fuel Increase (Decrease) in Inventories Accounts receivable, unbilled revenues and other operating assets Increase (Decrease) in Accounts Receivable and Other Operating Assets Accounts payable and other operating liabilities Increase (Decrease) in Other Accounts Payable and Accrued Liabilities Regulatory assets - current Increase (Decrease) in Other Regulatory Assets Regulatory liabilities - current Increase (Decrease) in Regulatory Liabilities Contributions to defined benefit pension plans Payment for Pension Benefits Other operating activities, net Increase (Decrease) in Other Operating Assets and Liabilities, Net Net cash provided by operating activities of continuing operations Net Cash Provided by (Used in) Operating Activities, Continuing Operations Net cash provided by (used in) operating activities of discontinued operations Cash Provided by (Used in) Operating Activities, Discontinued Operations Net cash provided by operating activities Net Cash Provided by (Used in) Operating Activities Investing activities: Net Cash Provided by (Used in) Investing Activities [Abstract] Property, plant and equipment additions Payments to Acquire Property, Plant, and Equipment Purchase of investment Payments to Acquire Other Investments Other investing activities Payments for (Proceeds from) Other Investing Activities Net cash provided by (used in) investing activities of continuing operations Net Cash Provided by (Used in) Investing Activities, Continuing Operations Net cash provided by investing activities of discontinued operations Cash Provided by (Used in) Investing Activities, Discontinued Operations Net cash provided by (used in) investing activities Net Cash Provided by (Used in) Investing Activities Financing activities: Net Cash Provided by (Used in) Financing Activities [Abstract] Dividends paid on common stock Payments of Dividends Common stock issued Proceeds from Issuance of Common Stock Net (payments) borrowings of short-term debt Proceeds from (Repayments of) Short-term Debt Long-term debt - repayments Proceeds from Issuance of Long-term Debt Long-term debt - repayments Repayments of Long-term Debt Distributions to noncontrolling interest Payments to Noncontrolling Interests Other financing activities Proceeds from (Payments for) Other Financing Activities Net cash provided by (used in) financing activities Net Cash Provided by (Used in) Financing Activities Net change in cash, cash equivalents and restricted cash Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Cash, cash equivalents and restricted cash at beginning of period Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents Cash, cash equivalents and restricted cash at end of period Supplemental Cash Flow Information [Abstract] Non-cash Investing and Financing Activities from Continuing Operations [Abstract] Noncash Investing and Financing Items [Abstract] Property, plant and equipment acquired with accrued liabilities Capital Expenditures Incurred but Not yet Paid Supplemental Cash Flow Elements [Abstract] Supplemental Cash Flow Elements [Abstract] Interest (net of amounts capitalized) Interest Paid, Excluding Capitalized Interest, Operating Activities Income taxes Income Taxes Paid, Net Subsequent Events [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Segment Reporting Segment Reporting, Policy [Policy Text Block] Use of Estimates and Basis of Presentation Use of Estimates, Policy [Policy Text Block] Recently Issued and Adopted Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Statement of Comprehensive Income [Abstract] Statement [Table] Statement [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Interest Expense Interest Expense [Member] Cost of Sales Cost of Sales [Member] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Cash Flow Hedging Cash Flow Hedging [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member] Accumulated Defined Benefit Plans Adjustment Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Reclassification Out Of Accumulated Other Comprehensive Income [Axis] Reclassification out of Accumulated Other Comprehensive Income [Axis] Reclassification Out Of Accumulated Other Comprehensive Income [Domain] Reclassification out of Accumulated Other Comprehensive Income [Domain] Reclassification Out Of Accumulated Other Comprehensive Income Reclassification out of Accumulated Other Comprehensive Income [Member] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest rate swaps Interest Rate Swap [Member] Commodity Contract Commodity Contract [Member] Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items] Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items] Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax benefit (expense) Income Tax Expense (Benefit) Operations and maintenance Operating Expense, Maintenance, Operations, and Other Costs and Expense Amount of operating expenses for the period including selling, general, and administrative expenses, costs include routine repairs and maintenance and operations. Derivative Instruments and Hedging Activities Disclosure [Abstract] Risk Management Activities Derivative Instruments and Hedging Activities Disclosure [Text Block] Debt Disclosure [Abstract] Notes Payable, Current Maturities and Debt Debt Disclosure [Text Block] Defined Benefit Plan [Abstract] Employee Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Materials, Supplies and Fuel Inventory Disclosure [Text Block] Investments, All Other Investments [Abstract] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Valuation, Income Approach Valuation, Income Approach [Member] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Investment in privately held oil and gas company Cost-method Investments [Member] Cash surrender value of life insurance contracts Cash Surrender Value [Member] Total investments Investments [Member] Statement [Line Items] Statement [Line Items] Assets Held For Sale Used to Acquire Other Investments Assets Held For Sale Used to Acquire Other Investments Assets Held For Sale Used to Acquire Other Investments Discount Rate Used in Valuation of Fair Value of Oil and Gas Reserve Quantities Discount Rate Used in Valuation of Fair Value of Oil and Gas Reserve Quantities Discount Rate Used in Valuation of Fair Value of Oil and Gas Reserve Quantities Investments Long-term Investments Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Regulatory Assets and Liabilities Disclosure [Abstract] Schedule of Regulatory Assets and Liabilities Schedule of Regulatory Assets and Liabilities [Table Text Block] The entire disclosure for detailed information about regulatory assets and liabilities, including current and noncurrent assets created when regulatory agencies permit the deferral of costs to the balance sheet that would otherwise be required to appear on the company's income statement and would be charged against current expenses or revenues, as well as current and noncurrent liabilities created when regulatory agencies permit. [Table Text Block] Reclassification adjustment of benefit plan - prior service cost, (tax) benefit Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax Net unrealized gains/losses on commodity derivatives, (tax) benefit Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Reclassification of net realized gains/losses, (tax) benefit Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax Earnings Per Share [Abstract] Net income available for common stock Net Income (Loss) Attributable to Parent Weighted average shares - basic (in shares) Weighted Average Number of Shares Outstanding, Basic Dilutive effect of: Dilutive Securities, Effect on Basic Earnings Per Share [Abstract] Equity Units (in shares) Incremental Common Shares Attributable to Dilutive Effect of Equity Unit Purchase Agreements Equity compensation (in shares) Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements Weighted average shares - diluted (in shares) Weighted Average Number of Shares Outstanding, Diluted Statement of Financial Position [Abstract] ASSETS Assets [Abstract] Current assets: Assets, Current [Abstract] Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Restricted cash Restricted Cash and Investments, Current Accounts receivable, net Accounts Receivable, Net, Current Materials, supplies and fuel Inventory, Net Derivative assets, current Derivative Asset, Current Income tax receivable, net Income Taxes Receivable, Current Regulatory assets, current Regulatory Assets, Current Other current assets Other Assets, Current Total current assets Assets, Current Property, plant and equipment Property, Plant and Equipment, Gross Less: accumulated depreciation and depletion Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Total property, plant and equipment, net Property, Plant and Equipment, Net Other assets: Other Assets, Noncurrent [Abstract] Goodwill Goodwill Intangible assets, net Intangible Assets, Net (Excluding Goodwill) Regulatory assets, non-current Regulatory Assets, Noncurrent Other assets, non-current Other Assets, Noncurrent Total other assets, non-current Assets, Noncurrent, Other than Noncurrent Investments and Property, Plant and Equipment TOTAL ASSETS Assets LIABILITIES AND TOTAL EQUITY Liabilities and Equity [Abstract] Current liabilities: Liabilities, Current [Abstract] Accounts payable Accounts Payable, Current Accrued liabilities Accrued Liabilities, Current Derivative liabilities, current Derivative Liability, Current Regulatory liabilities, current Regulatory Liability, Current Notes payable Short-term Debt Current maturities of long-term debt Long-term Debt, Current Maturities Total current liabilities Liabilities, Current Long-term debt Long-term Debt, Excluding Current Maturities Deferred credits and other liabilities: Liabilities, Noncurrent [Abstract] Deferred income tax liabilities, net Deferred Tax Liabilities, Net, Noncurrent Regulatory liabilities, non-current Regulatory Liability, Noncurrent Benefit plan liabilities Liability, Defined Benefit Plan, Noncurrent Other deferred credits and other liabilities Other Liabilities, Noncurrent Total deferred credits and other liabilities Liabilities, Other than Long-term Debt, Noncurrent Commitments and contingencies (See Notes 8, 10, 15, 16) Commitments and Contingencies Equity: Stockholders' Equity Attributable to Parent [Abstract] Common stock $1 par value; 100,000,000 shares authorized; issued 61,480,640 and 60,048,567 shares, respectively Common Stock, Value, Issued Additional paid-in capital Additional Paid in Capital Retained earnings Retained Earnings (Accumulated Deficit) Treasury stock, at cost – 26,572 and 44,253 shares, respectively Treasury Stock, Value Accumulated other comprehensive income (loss) Accumulated Other Comprehensive Income (Loss), Net of Tax Total stockholders’ equity Stockholders' Equity Attributable to Parent Noncontrolling interest Noncontrolling Interest in Variable Interest Entity Total equity Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest TOTAL LIABILITIES AND TOTAL EQUITY Liabilities and Equity Segment Reporting Information, Additional Information [Abstract] Reconciliation of Assets from Segment to Consolidated [Table] Reconciliation of Assets from Segment to Consolidated [Table] Scenario [Axis] Scenario [Axis] Scenario, Unspecified [Domain] Scenario, Unspecified [Domain] Recasting of Intercompany Capital Lease Scenario, Adjustment [Member] Segments [Axis] Segments [Axis] Segment [Domain] Segments [Domain] Electric Utilities Electric Utilities [Member] Electric Utilities Segment [Member] Gas Utilities Gas Utilities [Member] Gas Utilities [Member] Power Generation Power Generation [Member] Power Generation Mining Mining [Member] Mining Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Corporate Corporate, Non-Segment [Member] Segment Reporting, Asset Reconciling Item [Line Items] Segment Reporting, Asset Reconciling Item [Line Items] Assets Changes To Our Segment Performance Measure - Assets Changes To Our Segment Performance Measure - Assets Changes To Our Segment Performance Measure - Assets Payables and Accruals [Abstract] Accrued employee compensation, benefits and withholdings Employee-related Liabilities, Current Accrued property taxes Accrual for Taxes Other than Income Taxes, Current Customer deposits and prepayments Customer Deposits and Prepayments Accrued Liabilities - Customer Deposits and Prepayments Accrued interest and contract adjustment payments Interest Payable, Current Other (none of which is individually significant) Other Accrued Liabilities, Current Total accrued liabilities Subsequent Event [Table] Subsequent Event [Table] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Senior Unsecured Notes Due 2029 Senior Unsecured Notes Due 2029 [Member] Senior Unsecured Notes Due 2029 [Member] Senior Unsecured Notes Due 2049 Senior Unsecured Notes Due 2049 [Member] Senior Unsecured Notes Due 2049 [Member] Corporate Term Loan Due June 2021 Corporate Term Loan Due June 2021 [Member] Corporate Term Loan Due June 2021 [Member] Senior Unsecured Notes Due 2020 Senior Unsecured Notes Due 2020 [Member] Senior Unsecured Notes Due 2020 [Member] Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] Subsequent Event [Line Items] Subsequent Event [Line Items] Proceeds from Issuance of Debt Proceeds from Issuance of Debt Debt Instrument, Interest Rate, Stated Percentage Debt Instrument, Interest Rate, Stated Percentage Extinguishment of Debt, Amount Extinguishment of Debt, Amount Leases [Abstract] Operating Leases Operating Lease Liabilities, Payments Due [Abstract] 2019 Lessee, Operating Lease, Liability, Payments, Remainder of Fiscal Year 2020 Lessee, Operating Lease, Liability, Payments, Due Year Two 2021 Lessee, Operating Lease, Liability, Payments, Due Year Three 2022 Lessee, Operating Lease, Liability, Payments, Due Year Four 2023 Lessee, Operating Lease, Liability, Payments, Due Year Five Thereafter Lessee, Operating Lease, Liability, Payments, Due after Year Five Total lease payments Lessee, Operating Lease, Liability, Payments, Due Less imputed interest Lessee, Operating Lease, Liability, Undiscounted Excess Amount Present Value of Lease Liabilities - Operating leases Operating Lease, Liability Finance Lease Finance Lease Liabilities, Payments, Due [Abstract] 2019 Finance Lease, Liability, Payments, Remainder of Fiscal Year 2020 Finance Lease, Liability, Payments, Due Year Two 2021 Finance Lease, Liability, Payments, Due Year Three 2022 Finance Lease, Liability, Payments, Due Year Four 2023 Finance Lease, Liability, Payments, Due Year Five Thereafter Finance Lease, Liability, Payments, Due after Year Five Total lease payments Finance Lease, Liability, Payments, Due Less imputed interest Finance Lease, Liability, Undiscounted Excess Amount Present value of lease liabilities - Finance Leases Finance Lease, Liability Operating and Finance Lease Total Operating and Finance Lease Total [Abstract] Operating and Finance Lease Total [Abstract] 2019 Lease Liability Payments Remainder of Fiscal Year Lease Liability Payments Remainder of Fiscal Year 2020 Lease Liability Payments Due Year Two Lease Liability Payments Due Year Two 2021 Lease Liability Payments Due Year Three Lease Liability Payments Due Year Three 2022 Lease Liability Payments Due Year Four Lease Liability Payments Due Year Four 2023 Lease Liability Payments Due Year Five Lease Liability Payments Due Year Five Thereafter Lease Liability Payments After Year Five Lease Liability Payments After Year Five Total lease payments Lease Liability Payments Due Lease Liability Payments Due Less imputed interest Lease Liability Undiscounted Excess Amount Lease Liability Undiscounted Excess Amount Present value of lease liabilities Lease Liability Lease Liability Prior Year End Operating Lease Future Minimum Payments Prior Year End Operating Lease Future Minimum Payments [Abstract] Prior Year End Operating Lease Future Minimum Payments [Abstract] 2019 Operating Leases, Future Minimum Payments Due, Next Twelve Months 2020 Operating Leases, Future Minimum Payments, Due in Two Years 2021 Operating Leases, Future Minimum Payments, Due in Three Years 2022 Operating Leases, Future Minimum Payments, Due in Four Years 2023 Operating Leases, Future Minimum Payments, Due in Five Years Thereafter Operating Leases, Future Minimum Payments, Due Thereafter Total lease payments Operating Leases, Future Minimum Payments Due Reclassification Out of Accumulated Other Comprehensive Income (Loss) Reclassification out of Accumulated Other Comprehensive Income [Table Text Block] Schedule of Accumulated Other Comprehensive Income (Loss) Schedule of Accumulated Other Comprehensive Income (Loss) [Table Text Block] Earnings Per Share Earnings Per Share [Text Block] Commitments and Contingencies Disclosure [Abstract] Unrecorded Unconditional Purchase Obligation [Table] Unrecorded Unconditional Purchase Obligation [Table] Long-term Contract for Purchase of Electric Power [Axis] Long-term Contract for Purchase of Electric Power [Axis] Long-term Contract for Purchase of Electric Power [Domain] Long-term Contract for Purchase of Electric Power [Domain] Platte River Power Authority Wind Power Agreement Platte River Power Authority Wind Power Agreement [Member] Platte River Power Authority Wind Power Agreement [Member] Platte River Power Authority Platte River Power Authority [Member] Platte River Power Authority [Member] Category of Item Purchased [Axis] Category of Item Purchased [Axis] Long-term Purchase Commitment, Category of Item Purchased [Domain] Long-term Purchase Commitment, Category of Item Purchased [Domain] Power Purchase Agreements Power Purchase Agreements [Member] Power Purchase Agreements [Member] Unrecorded Unconditional Purchase Obligation [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Utility Plant, Megawatt Capacity Number of Megawatts Capacity Purchased Number of Megawatts being purchased under capacity and energy contract 2019 Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months 2020 Unrecorded Unconditional Purchase Obligation, Due within Two Years 2021 Unrecorded Unconditional Purchase Obligation, Due within Three Years 2022 Unrecorded Unconditional Purchase Obligation, Due within Four Years 2023 Unrecorded Unconditional Purchase Obligation, Due within Five Years Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Revenue from Contract with Customer [Abstract] Disaggregation of Revenue [Table] Disaggregation of Revenue [Table] Inter-company Revenues Intersegment Eliminations [Member] Customer [Axis] Customer [Axis] Customer [Domain] Customer [Domain] Retail - Electric , Natural Gas and Coal Retail - Electric , Natural Gas and Coal [Member] Retail - Electric , Natural Gas and Coal Transportation Transportation [Member] Transportation [Member] Wholesale Wholesale [Member] Wholesale [Member] Market - off-system sales Market Off-System Sales [Member] Market Off-System Sales [Member] Transmission/Other Transmission/Other [Member] Transmission/Other [Member] Product and Service [Axis] Product and Service [Axis] Product and Service [Domain] Product and Service [Domain] Other revenues Other [Member] Other [Member] Timing of Transfer of Good or Service [Axis] Timing of Transfer of Good or Service [Axis] Timing of Transfer of Good or Service [Domain] Timing of Transfer of Good or Service [Domain] Services transferred at a point in time Transferred at Point in Time [Member] Services transferred over time Transferred over Time [Member] Disaggregation of Revenue [Line Items] Disaggregation of Revenue [Line Items] Revenue from contracts with customers Revenue from Contract with Customer, Excluding Assessed Tax Total revenues Revenues Changes To Our Segment Performance Measure - Revenues Changes To Our Segment Performance Measure - Revenues Changes To Our Segment Performance Measure - Revenues Supplemental Disclosure of Cash Flow Information Cash Flow, Supplemental Disclosures [Text Block] Business Segment Information Segment Reporting Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value Measurements Derivatives and Fair Value [Text Block] Accrued Liabilities Accounts Payable and Accrued Liabilities Disclosure [Text Block] Schedule Of Regulatory Assets And Liabilities [Table] Schedule Of Regulatory Assets And Liabilities [Table] A table of assets and liabilities that are created when regulatory agencies permit public utilities to defer recognitiion of certain revenues or costs included in rate setting. Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Asset [Domain] Regulatory Asset [Domain] Deferred energy and fuel cost adjustments Deferred Fuel Costs [Member] Deferred gas cost adjustments Asset Recoverable Gas Costs [Member] Gas price derivatives Price Risk Derivative [Member] Deferred taxes on AFUDC Allowance For Funds Used During Construction [Member] The offsetting entry to other income when an enterprise capitalizes, as part of the cost of plant and equipment, the cost of financing construction as financed partially by equity. Employee benefit plans Pension and Other Postretirement Plans Costs [Member] Environmental Environmental [Member] Environmental costs associated with utility property [Member] Loss on reacquired debt Loss on Reacquired Debt [Member] Renewable energy standard adjustment Renewable Energy Program [Member] Deferred taxes on flow through accounting Flow Through Accounting [Member] The income tax effects of certain tax items are reflected in cost of service for the customer in which the tax benefits are realized and result in lower utiity rates. The regulatory treatment under flow-through accounting was applied for this tax benefit generated by utility costs that were previously capitalized for tax purposes in a rate case settlement and resulted in a lower rate increase. A regulatory asset was established to reflect the future increase in taxes payable recovered from sutomers as the temporarty difference reverses.[Member] Decommissioning costs Environmental Restoration Costs [Member] Gas supply contract termination Gas Supply Contract Termination [Member] Gas Supply Contract Termination [Member] Other regulatory assets Other Regulatory Assets [Member] Other regulatory assets [Member] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Deferred energy and gas costs Cost of removal Removal Costs [Member] Excess deferred income taxes Deferred Income Tax Charges a [Member] Rate Action of a regulator resulting in capitalization or accrual of income taxes TCJA revenue reduction Revenue Subject to Refund [Member] Other regulatory liabilities Other Regulatory Liabilities [Member] Other regulatory liabilities [Member] Schedule Of Regulatory Assets And Liabilities [Line Items] Schedule Of Regulatory Assets And Liabilities [Line Items] Schedule of regulatory asset and liabilities line items Regulatory assets Net Regulatory Assets [Abstract] Regulatory assets Regulatory Assets Less current regulatory assets Regulatory liabilities Regulatory Assets and Liabilities, Other Disclosures [Abstract] Regulatory liabilities Regulatory Liabilities Less current regulatory liabilities Management's Statement Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Derivative [Table] Derivative [Table] Energy [Axis] Energy [Axis] Energy [Domain] Energy [Domain] Natural Gas, Distribution Natural Gas, Distribution [Member] Natural Gas for distribution [Member] Natural gas futures purchased Future [Member] Natural gas options purchased, net Commodity Option [Member] Natural gas basis swaps purchased Basis Swap [Member] Natural gas over-the-counter swaps, net Fixed for Float Swaps Purchased [Member] Fixed for Float Swaps Purchased [Member] Natural gas physical contracts, net Natural Gas Physical Purchases [Member] Natural Gas Physical Purchases [Member] Derivative [Line Items] Derivative [Line Items] Derivative, Nonmonetary Notional Amount Derivative, Nonmonetary Notional Amount Derivative, Remaining Maturity Derivative, Remaining Maturity Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Credit Risk Derivative Liabilities, at Fair Value Collateral Already Posted, Aggregate Fair Value Lessee, Lease, Description [Table] Lessee, Lease, Description [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Lessee, Lease, Description [Line Items] Lessee, Lease, Description [Line Items] Lessee - Lease Term Lessee - Lease Term Lessee - Lease Term Restructuring Type [Axis] Restructuring Type [Axis] Type of Restructuring [Domain] Type of Restructuring [Domain] Legal Entity Restructuring Other Restructuring [Member] Income tax (expense) benefit associated with changes in the prior estimated impact of tax reform on deferred income taxes Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Deferred Income Tax (Expense) Benefit Effective Tax Rate Effective Income Tax Rate Reconciliation, Percent Production tax credits Effective Income Tax Rate Reconciliation, Deduction, Qualified Production Activity, Amount Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Black Hills Corporation Black Hills Corporation [Member] Black Hills Corporation [Member] Corporate Term Loan Due July 2020 Corporate Term Loan Due July 2020 [Member] Corporate Term Loan Due July 2020 [Member] Document and Entity Information [Abstract] Abstract model section heading for document and entity information Document Type Document Type Document Quarterly Report Document Quarterly Report Document Period End Date Document Period End Date Document Transition Report Document Transition Report Entity File Number Entity File Number Entity Registrant Name Entity Registrant Name Entity Incorporation, State or Country Code Entity Incorporation, State or Country Code Entity Tax Identification Number Entity Tax Identification Number Entity Address, Address Line One Entity Address, Address Line One Entity Address, City or Town Entity Address, City or Town Entity Address, State or Province Entity Address, State or Province Entity Address, Postal Zip Code Entity Address, Postal Zip Code City Area Code City Area Code Local Phone Number Local Phone Number Entity Current Reporting Status Entity Current Reporting Status Entity Interactive Data Current Entity Interactive Data Current Entity Filer Category Entity Filer Category Entity Small Business Entity Small Business Entity Emerging Growth Company Entity Emerging Growth Company Entity Shell Company Entity Shell Company Title of 12(b) Security Title of 12(b) Security Trading Symbol Trading Symbol Security Exchange Name Security Exchange Name Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Central Index Key Entity Central Index Key Amendment Flag Amendment Flag Document Fiscal Year Focus Document Fiscal Year Focus Document Fiscal Period Focus Document Fiscal Period Focus Current Fiscal Year End Date Current Fiscal Year End Date Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table] Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Equity compensation - (in shares) Stock Compensation Plan [Member] Restricted Stock (in shares) Restricted Stock [Member] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Anti-dilutive shares (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] External Operating Revenue External Customers [Member] External Customers Inter-company Operating Revenue Intercompany Customers [Member] Intercompany Customers [Member] Other Revenues Consolidation, Eliminations Consolidation, Eliminations [Member] Segment Reporting Information Segment Reporting Information [Line Items] Revenues Operating income Operating Income (Loss) Interest expense, net Interest Income (Expense), Net Impairment of investment Other Nonoperating Income (Expense) Including Allowance For Funds Used During Construction, Equity Other Nonoperating Income (Expense) Including Allowance For Funds Used During Construction, Equity Other Nonoperating Income (Expense) Including Allowance For Funds Used During Construction, Equity Income tax benefit (expense) Income from continuing operations Net (loss) from discontinued operations Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Noncontrolling Interest Net income available for common stock Net Income (Loss) Available to Common Stockholders, Basic Changes To Our Segment Performance Measure - Operating Income Changes To Our Segment Performance Measure - Operating Income Changes To Our Segment Performance Measure - Operating Income Operating Leases Lessor, Operating Lease, Payments, Fiscal Year Maturity [Abstract] 2019 Lessor, Operating Lease, Payments to be Received, Remainder of Fiscal Year 2020 Lessor, Operating Lease, Payments to be Received, Two Years 2021 Lessor, Operating Lease, Payments to be Received, Three Years 2022 Lessor, Operating Lease, Payments to be Received, Four Years 2023 Lessor, Operating Lease, Payments to be Received, Five Years Thereafter Lessor, Operating Lease, Payments to be Received, Thereafter Total lease receivables Lessor, Operating Lease, Payments to be Received Receivables [Abstract] Accounts Receivable Loans, Notes, Trade and Other Receivables Disclosure [Text Block] Other comprehensive income (loss), net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Reclassification adjustments of benefit plan liability - prior service cost (net of tax of $3, $10, $13 and $29, respectively) Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Reclassification of net realized (gains) losses Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Net unrealized gains (losses) on commodity derivatives (net of tax of $35, $0, $100 and $51, respectively) Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Other comprehensive income, net of tax Other Comprehensive Income (Loss), Net of Tax Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Less: comprehensive income attributable to noncontrolling interest Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest Comprehensive income available for common stock Comprehensive Income (Loss), Net of Tax, Attributable to Parent Other Comprehensive Income (Loss) Comprehensive Income (Loss) Note [Text Block] Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Regulatory Agency [Axis] Regulatory Agency [Axis] Regulatory Agency [Domain] Regulatory Agency [Domain] Wyoming Public Service Commission (WPSC) Wyoming Public Service Commission (WPSC) [Member] Wyoming Public Service Commission (WPSC) [Member] Wyoming Public Service Commission (WPSC) and South Dakota Public Utilities Commission (SDPUC) Wyoming Public Service Commission (WPSC) and South Dakota Public Utilities Commission (SDPUC) [Member] Wyoming Public Service Commission (WPSC) and South Dakota Public Utilities Commission (SDPUC) [Member] South Dakota Public Utilities Commission (SDPUC) South Dakota Public Utilities Commission (SDPUC) [Member] South Dakota Public Utilities Commission (SDPUC) [Member] Kansas Corporation Commission Kansas Corporation Commission [Member] Kansas Corporation Commission Nebraska Public Service Commission (NPSC) Nebraska Public Service Commission (NPSC) [Member] Nebraska Public Service Commission (NPSC) [Member] Colorado Public Utilities Commission (CPUC) Colorado Public Utilities Commission (CPUC) [Member] Colorado Public Utilities Commission (CPUC) [Member] Black Hills Energy, Wyoming Gas Black Hills Energy, Wyoming Gas [Member] Black Hills Energy, Wyoming Gas [Member] South Dakota Electric and Wyoming Electric South Dakota Electric and Wyoming Electric [Member] South Dakota Electric and Wyoming Electric [Member] South Dakota Electric South Dakota Electric [Member] South Dakota Electric [Member] Black Hills Energy, Kansas Gas Black Hills Energy, Kansas Gas [Member] Black Hills Energy, Kansas Gas Black Hills Energy, Nebraska Gas Black Hills Energy, Nebraska Gas [Member] Black Hills Energy, Nebraska Gas [Member] Black Hills Energy, Colorado Gas Black Hills Energy, Colorado Gas [Member] Black Hills Energy, Colorado Gas [Member] Name of Property [Axis] Name of Property [Axis] Name of Property [Domain] Name of Property [Domain] Corriedale Wind Project Corriedale Wind Project [Member] Corriedale Wind Project [Member] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Axis] Public Utilities, Regulatory Proceeding [Domain] Public Utilities, Regulatory Proceeding [Domain] Rate Review Filed with the Regulatory Agency Rate Review Filed with the Regulatory Agency [Member] Rate Review Filed with the Regulatory Agency [Member] Customer Subscription Period Was Completed With Customer Interest Fulfilling All Available Energy Customer Subscription Period Was Completed With Customer Interest Fulfilling All Available Energy [Member] Customer Subscription Period Was Completed With Customer Interest Fulfilling All Available Energy [Member] Amount of Increase sought in Generating Capacity Under Environmental Improvement Adjustment Tariff Amount of Increase sought in Generating Capacity Under Environmental Improvement Adjustment Tariff [Member] Amount of Increase sought in Generating Capacity Under Environmental Improvement Adjustment Tariff [Member] New Total Sought In Generating Capacity Under Environmental Improvement Adjustment Tariff New Total Sought In Generating Capacity Under Environmental Improvement Adjustment Tariff [Member] New Total Sought In Generating Capacity Under Environmental Improvement Adjustment Tariff [Member] Application Filed Application Approved [Member] Application Approved Rate Review Expected To Be Filed In The Next Fiscal Year Rate Review Expected To Be Filed In The Next Fiscal Year [Member] Rate Review Expected To Be Filed In The Next Fiscal Year [Member] Public Utilities, General Disclosures [Line Items] Public Utilities, General Disclosures [Line Items] Public Utilities, Property, Plant and Equipment, Cost Of Plant Investment Public Utilities, Property, Plant and Equipment, Cost Of Plant Investment Public Utilities, Property, Plant and Equipment, Cost Of Plant Investment Utility Plant, Megawatt Capacity Utility Plant, Megawatt Capacity Utility Plant, Megawatt Capacity Public Utilities, Number of Electric Utilities Jointly Owning Wind Project Public Utilities, Number of Electric Utilities Jointly Owning Wind Project Public Utilities, Number of Electric Utilities Jointly Owning Wind Project Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Requested Rate Increase, Amount Public Utilities, Requested Rate Increase (Decrease), Amount Public Utilities, Number of Companies Being Merged Public Utilities, Number of Companies Being Merged Public Utilities, Number of Companies Being Merged Statement of Stockholders' Equity [Abstract] Equity Stockholders' Equity Note Disclosure [Text Block] Fair Value Measurements, Recurring and Nonrecurring [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] Measurement Frequency [Axis] Measurement Frequency [Axis] Fair Value, Measurement Frequency [Domain] Fair Value, Measurement Frequency [Domain] Fair Value, Measurements, Recurring Fair Value, Measurements, Recurring [Member] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1 Fair Value, Inputs, Level 1 [Member] Fair Value, Inputs, Level 2 Fair Value, Inputs, Level 2 [Member] Fair Value, Inputs, Level 3 Fair Value, Inputs, Level 3 [Member] Measurement Basis [Axis] Measurement Basis [Axis] Fair Value Measurement [Domain] Fair Value Measurement [Domain] Portion at Fair Value Measurement Portion at Fair Value Measurement [Member] Estimate of Fair Value Measurement Estimate of Fair Value Measurement [Member] Business Group [Axis] Business Group [Axis] Business Group (grouping of segments) [Axis] Business Group [Domain] Business Group [Domain] [Domain] for Business Group (grouping of segments) [Axis] Utilities Group Utilities Group [Member] Utilities Group - consists of our Electric Utilities and our Gas Utilities segments Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Derivative Assets, Commodity Derivatives Derivative Asset Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset Derivative Assets, Total Assets, Fair Value Disclosure Derivative Liabilities, Fair Value Disclosure Derivative Liability Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset Derivative Liability, Fair Value, Gross Asset and Right to Reclaim Cash, Offset Derivative Liabilities, Total Financial and Nonfinancial Liabilities, Fair Value Disclosure Black Hills Wyoming and Wyoming Electric Black Hills Wyoming and Wyoming Electric [Member] Black Hills Wyoming and Wyoming Electric [Member] Federal Energy Regulatory Commission (FERC) Federal Energy Regulatory Commission (FERC) [Member] Federal Energy Regulatory Commission (FERC) [Member] Pending FERC Approval - Number of Megawatts Capacity to be Purchased and Delivered Through Intercompany Agreement Pending FERC Approval - Number of Megawatts Capacity to be Purchased and Delivered Through Intercompany Agreement Pending FERC Approval - Number of Megawatts Capacity to be Purchased and Delivered Through Intercompany Agreement Unrecorded Unconditional Purchase Obligation, Term Unrecorded Unconditional Purchase Obligation, Term Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Other Nonoperating Income (Expense) Other Nonoperating Income (Expense) [Member] Derivative, Gain (Loss) on Derivative, Net Derivative, Gain (Loss) on Derivative, Net Regulatory assets Schedule of Short-term Debt Schedule of Short-term Debt [Table Text Block] Common Stock Common Stock [Member] At The Market Equity Offering Program Authorized Aggregate Value At The Market Equity Offering Program Authorized Aggregate Value At The Market Equity Offering Program authorized an aggregate value that the Company may, through its sales agents, offer and sell from time to time shares of its common stock. At The Market Equity Program Proceeds from Sale of Stock At The Market Equity Program Proceeds from Sale of Stock At The Market Equity Program Proceeds from Sale of Stock At The Market Equity Offering Program Shares Issued At The Market Equity Offering Program Shares Issued At The Market Equity Offering Program Shares Issued Payments of Stock Issuance Costs Payments of Stock Issuance Costs Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest [Abstract] Treasury Stock Treasury Stock [Member] Additional Paid-in Capital Additional Paid-in Capital [Member] Retained Earnings Retained Earnings [Member] Accumulated Other Comprehensive Income (loss) AOCI Attributable to Parent [Member] Noncontrolling Interest Noncontrolling Interest [Member] Increase (Decrease) in Stockholders' Equity [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Common Stock, Shares, Outstanding Common Stock, Shares, Outstanding Total equity Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest Net Income (Loss), Including Portion Attributable to Nonredeemable Noncontrolling Interest Other comprehensive income (loss), net of tax Dividends on Common Stock Dividends, Common Stock, Cash Share-based compensation, shares Stock Issued During Period, Shares, Share-based Compensation, Net of Forfeitures Share-based compensation Stock Issued During Period, Value, Share-based Compensation, Net of Forfeitures Dividend reinvestment and stock purchase plan, shares Stock Issued During Period, Shares, Dividend Reinvestment Plan Dividend reinvestment and stock purchase plan Stock Issued During Period, Value, Dividend Reinvestment Plan Other stock transactions Stockholders' Equity, Other Issuance of common stock, shares Stock Issued During Period, Shares, New Issues Issuance of common stock Stock Issued During Period, Value, New Issues Issuance costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Implementation of ASU 2016-02 Leases Cumulative Effect of New Accounting Principle in Period of Adoption Distributions to noncontrolling interest Noncontrolling Interest, Decrease from Distributions to Noncontrolling Interest Holders Common Stock, Shares, Outstanding Total equity Investments Investment [Text Block] Schedule of Fair Value Hierarchy, Measured on Recurring Basis Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block] Schedule of Derivative Instruments Balance Sheet Location Schedule of Derivative Instruments in Statement of Financial Position, Fair Value [Table Text Block] Operating cash flows from operating leases Operating Lease, Payments Operating cash flows from finance lease Finance Lease, Interest Payment on Liability Financing cash flows from finance lease Finance Lease, Principal Payments Operating leases Right-of-Use Asset Obtained in Exchange for Operating Lease Liability Finance lease Right-of-Use Asset Obtained in Exchange for Finance Lease Liability Fair Value of Financial Instruments Fair Value Disclosures [Text Block] Fair Value of Financial Instruments Fair Value, by Balance Sheet Grouping [Table Text Block] Operating leases Operating Lease, Weighted Average Remaining Lease Term Finance lease Finance Lease, Weighted Average Remaining Lease Term Operating leases Operating Lease, Weighted Average Discount Rate, Percent Finance lease Finance Lease, Weighted Average Discount Rate, Percent Line of Credit Facility [Table] Line of Credit Facility [Table] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Consolidated Indebtedness to Capitalization Ratio Debt Instrument, Consolidated Indebtedness To Capitalization Ratio Requirement For The Next Fiscal Year Debt Instrument, Consolidated Indebtedness To Capitalization Ratio Requirement For The Next Fiscal Year Segment Reporting Schedule of Segment Reporting Information, by Segment [Table Text Block] Reconciliation of Assets from Segment to Consolidated Reconciliation of Assets from Segment to Consolidated [Table Text Block] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Balance Sheet Location [Axis] Balance Sheet Location [Axis] Balance Sheet Location [Domain] Balance Sheet Location [Domain] Derivative assets — current Derivative Assets, Current [Member] Line item in the statement of financial position in a classified presentation in which the fair value amounts of the derivative instruments are included. Other assets, non-current Derivative Assets, Noncurrent [Member] Line item in the statement of financial position in a classified presentation in which the fair value amounts of the derivative instruments are included. Derivative liabilities — current Derivative Liabilities, Current [Member] Line item in the statement of financial position in a classified presentation in which the fair value amounts of the derivative instruments are included. Other deferred credits and other liabilities Derivative Liabilities, Noncurrent [Member] Line item in the statement of financial position in a classified presentation in which the fair value amounts of the derivative instruments are included. Derivatives, Carrying Amount and Fair Value [Line Items] Derivatives, Fair Value [Line Items] Derivative Asset, Fair Value, Gross Asset Derivative Asset, Fair Value, Gross Asset Derivative Liability, Fair Value, Gross Liability Derivative Liability, Fair Value, Gross Liability Derivative Asset, Fair Value, Net Derivative Asset, Fair Value, Amount Not Offset Against Collateral Discontinued Operations and Disposal Groups [Abstract] Discontinued Operations Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Operating lease cost Operating Lease, Cost Amortization of right-of-use asset Finance Lease, Right-of-Use Asset, Amortization Interest on lease liabilities Finance Lease, Interest Expense Total lease cost Lease, Cost Schedule of Earnings Per Share Reconciliation Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Income Statement [Abstract] Operating expenses: Operating Expenses [Abstract] Fuel, purchased power and cost of natural gas sold Fuel, Purchased Power and Cost of Gas Sold Fuel costs incurred that are directly related to goods produced and sold and services rendered during the reporting period, combined with the amount of purchased power charged against earnings for the period, and cost of gas sold to customers for the period. Taxes - property and production Taxes, Miscellaneous Total operating expenses Operating Expenses Operating income Other income (expense): Nonoperating Income (Expense) [Abstract] Interest charges - Interest and Debt Expense [Abstract] Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts) Interest Costs Incurred Allowance for funds used during construction - borrowed Public Utilities Allowance For Funds Used During Construction Borrowed Interest Costs Capitalized The component of the allowance for funds used during construction during the period which represents the approximate composite costs of borrowed funds and is netted against interest expense. Interest income Investment Income, Interest Allowance for funds used during construction - equity Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity Other income (expense), net Other Nonoperating Income (Expense) Total other income (expense) Nonoperating Income (Expense) Income before income taxes Net income Net income available for common stock Net income from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent Earnings per share of common stock: Earnings Per Share, Basic [Abstract] Earnings Per Share, Basic [Abstract] Income from continuing operations, Basic (usd per share) Income (Loss) from Continuing Operations, Per Basic Share (Loss) from discontinued operations, Basic (usd per share) Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share Earnings (loss) per share, Basic (usd per share) Earnings Per Share, Basic Earnings Per Share, Diluted [Abstract] Earnings Per Share, Diluted [Abstract] Income from continuing operations, Diluted (usd per share) Income (Loss) from Continuing Operations, Per Diluted Share (Loss) from discontinued operations, Diluted (usd per share) Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share Earnings (loss) per share, Diluted (usd per share) Earnings Per Share, Diluted Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract] Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract] Weighted average number of shares of common stock, basic and diluted[Abstract] Basic (in shares) Diluted (in shares) Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Schedule of Accounts, Notes, Loans and Financing Receivable [Table] Accounts Receivable [Line Items] Accounts, Notes, Loans and Financing Receivable [Line Items] Accounts Receivable, Trade Accounts Receivable, Gross, Current Unbilled Receivables, Current Unbilled Receivables, Current Allowance for Doubtful Accounts Allowance for Doubtful Accounts Receivable, Current Accounts receivable, net New Accounting Pronouncements or Change in Accounting Principle [Table] New Accounting Pronouncements or Change in Accounting Principle [Table] Adjustments for New Accounting Pronouncements [Axis] Adjustments for New Accounting Pronouncements [Axis] Type of Adoption [Domain] Type of Adoption [Domain] Leases, ASU 2016-02 Accounting Standards Update 2016-02 [Member] New Accounting Pronouncements or Change in Accounting Principle [Line Items] New Accounting Pronouncements or Change in Accounting Principle [Line Items] Operating lease - right of use assets Operating Lease, Right-of-Use Asset Operating lease obligation liability Accrued rent receivable Deferred Rent Receivables, Net Cumulative effect of the adoption, net of tax impact Segment Reporting [Abstract] Energy and Capacity to Colorado Electric from Colorado IPP Energy and Capacity to Colorado Electric from Colorado IPP [Member] Energy and Capacity to Colorado Electric from Colorado IPP [Member] Lessor, Lease, Description [Table] Lessor, Lease, Description [Table] Lessor, Lease, Description [Line Items] Lessor, Lease, Description [Line Items] Lessor - Lease Term Lessor, Operating Lease, Term of Contract Operating lease income Operating Lease, Lease Income Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Lessee Disclosure [Abstract] Lease, Cost Lease, Cost [Table Text Block] Lessee - Supplemental Balance Sheet Information Lessee Supplemental Balance Sheet Information Related to Leases [Table Text Block] Lessee Supplemental Balance Sheet Information Related to Leases [Table Text Block] Lessee - Supplemental Cash Flow Information Lessee Supplemental Cash Flow Information Related to Leases [Table Text Block] Lessee Supplemental Cash Flow Information Related to Leases [Table Text Block] Lessee - Supplemental Weighted Average Schedule Lessee Supplemental Weighted Average Schedule Related to Leases [Table Text Block] Lessee Supplemental Weighted Average Schedule Related to Leases [Table Text Block] Lessee - Finance Lease, Liability, Maturity Finance Lease, Liability, Maturity [Table Text Block] Lessee, Operating Lease, Liability, Maturity Lessee, Operating Lease, Liability, Maturity [Table Text Block] Lessor Disclosure [Abstract] Operating Lease, Lease Income Operating Lease, Lease Income [Table Text Block] Lessor - Operating and Finance Lease, Liability, Maturity Schedule Of Maturities of Operating And Finance Lease Receivable [Table Text Block] Schedule Of Maturities of Operating And Finance Lease Receivable [Table Text Block] Common Stock, Dividends, Per Share, Cash Paid Common Stock, Dividends, Per Share, Cash Paid Revenue Revenue from Contract with Customer [Text Block] Materials and supplies Inventory, Raw Materials and Supplies, Gross Fuel - Electric Utilities Energy Related Inventory, Other Fossil Fuel Natural gas in storage held for distribution Energy Related Inventory, Natural Gas in Storage Total materials, supplies and fuel Interest expense incurred net of amounts capitalized (including amortization of debt issuance costs, premiums and discounts) Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, before Tax Schedule of Net Benefit Costs Schedule of Net Benefit Costs [Table Text Block] Schedule of Defined Benefit Plans Contributions Schedule of Defined Benefit Plans Contributions [Table Text Block] Schedule of Defined Benefit Plans Contributions for the current year and specific future years.[Text Block] Contract or notional amounts and terms of marketing activities and derivative commodity instruments Schedule of Notional Amounts of Outstanding Derivative Positions [Table Text Block] Derivative Instruments, Gain (Loss) Derivative Instruments, Gain (Loss) [Table Text Block] Schedule of Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Regulatory Accounting Schedule of Regulatory Assets and Liabilities [Text Block] Common Stock, Par Value (usd per share) Common Stock, Par or Stated Value Per Share Common Stock, Shares Issued Common Stock, Shares, Issued Treasury Stock, Shares Treasury Stock, Shares Common Stock, Shares Authorized Common Stock, Shares Authorized Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Interest Rate Swaps Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] AOCI Attributable to Parent, Net of Tax [Roll Forward] Accumulated Other Comprehensive Income [Roll Forward] Accumulated Other Comprehensive Income [Roll Forward] Accumulated Other Comprehensive Income (Loss), Net of Tax, Period Start Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Reclassification from AOCI, Current Period, Tax Reclassification from AOCI, Current Period, Tax Accumulated Other Comprehensive Income (Loss), Net of Tax, Period End Accounts Receivable and Allowance for Doubtful Accounts Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Schedule of Short-term Debt [Table] Schedule of Short-term Debt [Table] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility Revolving Credit Facility [Member] Short-term Debt, Type [Axis] Short-term Debt, Type [Axis] Short-term Debt, Type [Domain] Short-term Debt, Type [Domain] Commercial Paper Commercial Paper [Member] Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] Base Rate Base Rate [Member] Eurodollar Eurodollar [Member] Letter of Credit Letter of Credit [Member] Short-term Debt [Line Items] Short-term Debt [Line Items] Letters of Credit Letters of Credit Outstanding, Amount Current borrowing capacity Line of Credit Facility, Current Borrowing Capacity Number of one-year extension options Number Of One-Year Extension Options Number Of One-Year Extension Options Debt instrument, term Debt Instrument, Term Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Interest rate Line of Credit Facility, Interest Rate at Period End Commitment fee Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Commercial paper, maximum borrowing capacity Commercial Paper, Maximum Borrowing Capacity Commercial Paper, Maximum Borrowing Capacity Short-term interest rate Short-term Debt, Weighted Average Interest Rate, at Point in Time Supplemental Disclosure of Cash Flow Information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Schedule of Unconditional Purchase Obligations Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Subsequent Events Subsequent Events [Text Block] Operating lease assets Finance lease assets Finance Lease, Right-of-Use Asset Total lease assets Operating and Finance Lease Assets Operating and Finance Lease Assets Operating lease liability, current Operating Lease, Liability, Current Finance lease liability, current Finance Lease, Liability, Current Operating lease liability, non-current Operating Lease, Liability, Noncurrent Finance lease liability, non-current Finance Lease, Liability, Noncurrent Total lease liabilities Operating and Finance Lease Liabilities Operating and Finance Lease Liabilities Investments Investment [Table Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Carrying Amount Reported Value Measurement [Member] Fair Value Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Long-term debt, including current maturities Long-term Debt Long-term debt, including current maturities, Fair Value Long-term Debt, Fair Value Leases Lessee, Operating Leases [Text Block] Leases Lessee, Finance Leases [Text Block] Leases Lessor, Operating Leases [Text Block] Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Defined Benefit Pension Plans Pension Plan [Member] Other Postretirement Benefits Plan Other Postretirement Benefits Plan [Member] Supplemental Non-qualified Defined Benefit and Defined Contribution Plans Supplemental Employee Retirement Plan [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on plan assets Defined Benefit Plan, Expected Return (Loss) on Plan Assets Prior service cost (benefit) Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Net loss (gain) Defined Benefit Plan, Amortization of Gain (Loss) Net periodic benefit cost Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Payment for Pension and Other Postretirement Benefits [Abstract] Payment for Pension and Other Postretirement Benefits [Abstract] Contributions by Employer Defined Benefit Plan, Plan Assets, Contributions by Employer Estimated Future Employer Contributions in Current Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Remainder of Fiscal Year Estimated Future Employer Contributions in Next Fiscal Year Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year