Derivative Instruments and Hedging Activities Disclosure [Abstract] 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] Commitments and Contingencies Disclosure [Abstract] Unrecorded Unconditional Purchase Obligation [Table] Unrecorded Unconditional Purchase Obligation [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [Domain] Power purchased Power Purchased [Member] Power Purchased [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] Pueblo Airport Generation Pueblo Airport Generation [Member] Jointly Owned Pueblo Airport Generating Facility [Member] Unrecorded Unconditional Purchase Obligation [Line Items] Unrecorded Unconditional Purchase Obligation [Line Items] Lease Expiration Date Lease Expiration Date Number of Megawatts Capacity Purchased Number of Megawatts Capacity Purchased Number of Megawatts being purchased under capacity and energy contract Discontinued Operations and Disposal Groups [Abstract] Discontinued Operations Disposal Groups, Including Discontinued Operations, Disclosure [Text Block] Organization, Consolidation and Presentation of Financial Statements [Abstract] Business Description Segment Reporting, Policy [Policy Text Block] Use of Estimates and Basis of Presentation Use of Estimates, Policy [Policy Text Block] Principles of Consolidation Consolidation, Policy [Policy Text Block] Variable Interest Entities Consolidation, Variable Interest Entity, Policy [Policy Text Block] Cash and Cash Equivalents and Restricted Cash Cash and Cash Equivalents, Restricted Cash and Cash Equivalents, Policy [Policy Text Block] Accounts Receivable and Allowance for Doubtful Accounts Trade and Other Accounts Receivable, Policy [Policy Text Block] Revenue Recognition Revenue Recognition, Policy [Policy Text Block] Materials, Supplies and Fuel Inventory, Policy [Policy Text Block] Investments Investment, Policy [Policy Text Block] Property, Plant and Equipment Property, Plant and Equipment, Policy [Policy Text Block] Goodwill and Intangible Assets Goodwill and Intangible Assets, Policy [Policy Text Block] Asset Retirement Obligations Asset Retirement Obligation [Policy Text Block] Fair Value Measurements Fair Value of Financial Instruments, Policy [Policy Text Block] Derivatives and Hedging Activities Derivatives, Policy [Policy Text Block] Deferred Financing Costs Debt, Policy [Policy Text Block] Regulatory Accounting Public Utilities, Policy [Policy Text Block] Income Taxes Income Tax, Policy [Policy Text Block] Earnings per Share of Common Stock Earnings Per Share, Policy [Policy Text Block] Business Combinations Business Combinations Policy [Policy Text Block] Noncontrolling Interest Consolidation, Subsidiaries or Other Investments, Consolidated Entities, Policy [Policy Text Block] Share-Based Compensation Compensation Related Costs, Policy [Policy Text Block] Recently Issued and Adopted Accounting Standards New Accounting Pronouncements, Policy [Policy Text Block] Segment Reporting Information, Additional Information [Abstract] Reconciliation of Assets from Segment to Consolidated [Table] Reconciliation of Assets from Segment to Consolidated [Table] Segments [Axis] Segments [Axis] Reporting 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, Non-Segment Corporate, Non-Segment [Member] Disposal Group Classification [Axis] Disposal Group Classification [Axis] Disposal Group Classification [Domain] Disposal Group Classification [Domain] Discontinued Operations, Held-for-sale Discontinued Operations, Held-for-sale [Member] Segment Reporting, Asset Reconciling Item Segment Reporting, Asset Reconciling Item [Line Items] Total Assets Assets Income Tax Disclosure [Abstract] Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward] Liability for Uncertain Tax Positions, Noncurrent, Period Start Liability for Uncertainty in Income Taxes, Noncurrent Additions for prior year tax positions Unrecognized Tax Benefits, Increase Resulting from Prior Period Tax Positions Reductions for prior year tax positions Unrecognized Tax Benefits, Decrease Resulting from Prior Period Tax Positions Additions for current year tax positions Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions Settlements Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities Liability for Uncertain Tax Positions, Noncurrent, Period End Unrecognized Tax Benefits that Would Impact Effective Tax Rate Unrecognized Tax Benefits that Would Impact Effective Tax Rate Defined Benefit Plan [Abstract] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Retirement Plan Type [Axis] Retirement Plan Type [Axis] Retirement Plan Type [Domain] Retirement Plan Type [Domain] Pension Plans, Defined Benefit Pension Plan [Member] Supplemental Employee Retirement Plans, Defined Benefit Supplemental Employee Retirement Plan [Member] Other Postretirement Benefit Plans, Defined Benefit Other Postretirement Benefits Plan [Member] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] Accumulated Benefit Obligation Defined Benefit Plan, Accumulated Benefit Obligation Jointly Owned Utility Plant Interests [Table] Jointly Owned Utility Plant Interests [Table] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant [Domain] Jointly Owned Utility Plant [Domain] Wyodak Plant Wyodak Plant [Member] Wyodak Plant - power generation plant owned 20% by Black Hills Power and 80% by Pacificorp[Member] Transmission Tie Transmission Tie [Member] Jointly onwed Transmission Tie [Member] Wygen I Generating Facility Wygen I Generating Facility [Member] Jointly Owned Wygen I Generating Facility [Member] Wygen I I I Generating Facility Wygen I I I Generating Facility [Member] Wygen I I I - Black Hills Power's coal-fired power generation plant that commenced operations on April 1, 2010.[Member] Busch Ranch I Wind Farm Busch Ranch I Wind Farm [Member] Busch Ranch I Wind Farm [Member] Segments [Domain] Name of Property [Axis] Name of Property [Axis] Name of Property [Domain] Name of Property [Domain] West to East Transmission Tie West to East Transmission Tie [Member] West to East Transmission Tie [Member] East to West Transmission Tie East to West Transmission Tie [Member] East to West Transmission Tie [Member] Jointly Owned Utility Plant Interests [Line Items] Jointly Owned Utility Plant Interests [Line Items] Ownership Share Percentage Jointly Owned Utility Plant, Proportionate Ownership Share Utility Plant, Megawatt Capacity Utility Plant, Megawatt Capacity Utility Plant, Megawatt Capacity Plant in Service Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Construction Work in Progress Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Accumulated Depreciation Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Stockholders' Equity Note [Abstract] Schedule of Stock by Class [Table] Schedule of Stock by Class [Table] Plan Name [Axis] Plan Name [Axis] Plan Name [Domain] Plan Name [Domain] Dividend Reinvestment Plan Dividend Reinvestment and Stock Purchase Plan [Member] Dividend Reinvestment and Stock Purchase Plan [Member] Class of Stock [Line Items] Class of Stock [Line Items] Percent of recent average market price Common Stock Purchase Percentage of Recent Average Market Price Common stock purchase percentage of recent average market price Unissued Shares Available Common Stock, Capital Shares Reserved for Future Issuance Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Postretirement Health Coverage Postretirement Health Coverage [Member] Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Schedule of Allocation of Plan Assets Schedule of Allocation of Plan Assets [Table Text Block] Schedule of Defined Contribution Plans Contributions Schedule of Employer Contribution to Employee Benefit Plans [Table Text Block] Schedule of Employer Contribution to Defined Benefit and Defined Contribution Plans [Table Text Block] Schedule of Changes in Projected Benefit Obligations Schedule of Changes in Projected Benefit Obligations [Table Text Block] Schedule of Changes in Fair Value of Plan Assets Schedule of Changes in Fair Value of Plan Assets [Table Text Block] Schedule of Amounts Recognized in Balance Sheet Schedule of Amounts Recognized in Balance Sheet [Table Text Block] Schedule of Accumulated and Projected Benefit Obligations Schedule of Accumulated and Projected Benefit Obligations [Table Text Block] Components of net periodic benefit cost Schedule of Net Benefit Costs [Table Text Block] Schedule of Net Periodic Benefit Cost Not yet Recognized Schedule of Net Periodic Benefit Cost Not yet Recognized [Table Text Block] Schedule of Assumptions Used Schedule of Assumptions Used [Table Text Block] Schedule of Health Care Cost Trend Rates Schedule of Health Care Cost Trend Rates [Table Text Block] Schedule of Expected Benefit Payments Schedule of Expected Benefit Payments [Table Text Block] Guarantees [Abstract] Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Guarantor Obligations by Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Surety Bond Surety Bond [Member] Performance Guarantee Performance Guarantee [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Guarantor, Maximum Exposure Guarantor Obligations, Maximum Exposure, Undiscounted Asset Retirement Obligation Disclosure [Abstract] Asset Retirement Obligations Asset Retirement Obligation Disclosure [Text Block] Schedule of Components of Income Tax Expense (Benefit) Schedule of Components of Income Tax Expense (Benefit) [Table Text Block] Schedule of Deferred Tax Assets and Liabilities Schedule of Deferred Tax Assets and Liabilities [Table Text Block] Schedule of Effective Income Tax Rate Reconciliation Schedule of Effective Income Tax Rate Reconciliation [Table Text Block] Summary of Operating Loss Carryforwards Summary of Operating Loss Carryforwards [Table Text Block] Summary of Income Tax Contingencies Summary of Income Tax Contingencies [Table Text Block] Income Tax Contingency [Table] Income Tax Contingency [Table] Income Tax Authority [Axis] Income Tax Authority [Axis] Income Tax Authority [Domain] Income Tax Authority [Domain] State and Local Jurisdiction State and Local Jurisdiction [Member] Operating Loss Carryforwards [Line Items] Operating Loss Carryforwards [Line Items] Summary of State Tax Carryforwards Summary of Tax Credit Carryforwards [Table Text Block] Long-term Purchase Commitment [Table] Long-term Purchase Commitment [Table] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Unrecorded Unconditional Purchase Obligation by Category of Item Purchased [Axis] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Unconditional Purchase Obligation, Category of Goods or Services Acquired [Domain] Purchase Commitment Purchase Commitment [Member] Long-term Purchase Commitment [Line Items] Long-term Purchase Commitment [Line Items] Long Term Contract For Purchase of Fuel, Date of Contract Expiration Long Term Contract For Purchase of Fuel, Date of Contract Expiration Date of expiration of long-term contract to purchase commodities in use for utility operations including natural gas including transport Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Axis] Defined Benefit Plan, Plan Assets, Category [Domain] Defined Benefit Plan, Plan Assets, Category [Domain] Equity Securities Equity Securities [Member] Real Estate Real Estate [Member] Fixed Income Funds Fixed Income Funds [Member] Cash and Cash Equivalents Cash and Cash Equivalents [Member] Hedge Funds Hedge Funds, Multi-strategy [Member] Defined Benefit Plan, Target Allocation Percentage of Assets Defined Benefit Plan, Plan Assets, Target Allocation, Percentage Long-term Debt, Unclassified [Abstract] Deferred Finance Costs Remaining, Noncurrent Debt Issuance Costs, Noncurrent, Net Amortization expense for deferred financing costs Amortization of Debt Issuance Costs Equity Stockholders' Equity Note Disclosure [Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Performance Shares Performance Shares [Member] Performance Shares, Liability Awards Performance Shares, Liability Awards [Member] The liability components of equity Performance Shares awards [Member] Performance Shares, Equity Awards Performance Shares, Equity Awards [Member] The equity award components for Performance Shares [Member] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Share-based Compensation Arrangement by Share-based Payment Award [Line Items] Performance Share Award Payout, Cash Percentage Share Based Compensation Arrangement By Share Based Payment Award, Payout, Cash Percentage Percentage of performance based compensation to be paid out in cash. Performance Share Award Payout, Shares of Common Stock Percentage Share Based Compensation Arrangement By Share Based Payment Award, Payout, Shares Percentage Payout percentage for share based compensation to be issued in shares of common stock. The percentage paid in cash for the accrued equity portion of the performance share plan upon change in control The percentage paid in cash for the accrued equity portion of the performance share plan upon change in control The percentage paid in cash for the accrued equity portion of the performance share plan upon change in control Unrecognized compensation expense Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Performance Share Award, Percentage of Target Share-based Compensation Arrangement by Share-based Payment Award, Payout, Percentage of Target The percentage target for payout of equity compensation awards Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Outstanding [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Non-Option Equity Instruments, Outstanding [Roll Forward] Performance Shares, Number of Shares Authorized, Beginning of Period Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized Performance Shares, Granted in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period Performance Shares, Forfeited in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeited in Period Performance Shares, Vested in Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period Performance Shares, Number of Shares Authorized, End of Period Share-based Compensation Arrangement by Share-based Payment Award, Performance Shares, Outstanding [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Other Than Options, Weighted Average Price [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Other Than Options, Weighted Average Price [Roll Forward] Balance at beginning of period (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value Granted (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value Forfeited (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value Vested (usd per share) Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value Balance at end of period (usd per share) Blended volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Weighted Average Volatility Rate Historical volatility Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Volatility Rate Weighted Average Grant Date Fair Value (usd per share) Weighted Average Grant Date of Fair Value Share-Based Compensation Arrangement Weighted average fair value at grant date for non-vested equity-based awards issued during the period. Performance Goal - Percentile of Peer Group Performance Share-based Compensation Arrangement by Share Based Payment Award, Performance Goal - Percentile of Peer Group Performance Share-based Compensation Arrangement by Share Based Payment Award, Performance Goal - The Company's Total Shareholder Return for the Prescribed Period as a Percentile of Peer Group Performance Target shares, value Employee Service Share-based Compensation, Other Than Options, Total Payout Value The amount of the total payout for an employee service share-based compensation award for other than options that has been accrued at the Balance Sheet date. The total value includes the stock price, dividends and the percentage payout value. Unrecognized compensation expense Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Share-based Awards Other than Options Weighted-average recognition period Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Ownership [Axis] Ownership [Axis] Ownership [Domain] Ownership [Domain] Former SourceGas Noncontrolling Interest Former SourceGas Noncontrolling Interest [Member] Former SourceGas Noncontrolling Interest [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Source Gas Source Gas [Member] Source Gas [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock Common Stock [Member] Debt Instrument [Axis] Debt Instrument [Axis] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Remarketable Junior Subordinated Notes Due 2028 Remarketable Junior Subordinated Notes Due 2028 [Member] Remarketable Junior Subordinated Notes Due 2028 [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Acquisition Narrative [Abstract] Acquisition Narrative [Abstract] Acquisition Narrative [Abstract] Purchase Price Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Assumed long-term debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Ownership of subsidiary (percent) Subsidiary or Equity Method Investee, Cumulative Percentage Ownership after All Transactions Number of natural gas utilities acquired Number Of Natural Gas Utilities Acquired Number Of Natural Gas Utilities Acquired Number of customers served with acquisition Number Of Customers Acquired Number Of Customers Acquired Length of natural gas pipeline (miles) Length Of Natural Gas Pipeline Acquired Length Of Natural Gas Pipeline Acquired Cash consideration paid Business Combination, Consideration Transferred, Net of Long Term Debt and Cash Acquired, Before Working Capital Adjustment Business Combination, Consideration Transferred, Net of Long Term Debt and Cash Acquired, Before Working Capital Adjustment Proceeds from issuance of shares Proceeds From Issuance Of Common Stock and Sale Of Interest In Corporate Units Proceeds From Issuance Of Common Stock and Sale Of Interest In Corporate Units Issuance of common stock, shares Stock Issued During Period, Shares, New Issues Issuance of equity units Debt Instrument, Convertible, Number of Equity Instruments Long-term debt - issuance Proceeds from Issuance of Long-term Debt Pre-tax, incremental acquisition costs Business Combination, Acquisition Related Costs Revenue - Revenues Net income (loss) available for common stock Net Income (Loss) Available to Common Stockholders, Basic Cash consideration paid Business Combination, Consideration Transferred, Net of Long Term Debt and Cash Acquired Business Combination, Consideration Transferred, Net of Long Term Debt and Cash Acquired Goodwill Goodwill Less: Working capital adjustment received Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Working Capital Adjustment Received Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Working Capital Adjustment Received Expected tax deductible goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Acquisition Recap [Abstract] Acquisition Recap [Abstract] Acquisition Recap [Abstract] Less: Long-term debt assumed Consideration paid, net of working capital adjustment received Current Assets Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets Property, plant & equipment, net Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment Deferred charges and other assets, excluding goodwill Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets Current liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities Long-term debt Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Including Working Capital Changes Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt Including Working Capital Changes Deferred credits and other liabilities Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Other Total consideration paid, net of working-capital adjustment received Business Combination, Consideration Transferred Pro Forma Results Business Acquisition, Pro Forma Information [Abstract] Revenue Business Acquisition, Pro Forma Revenue Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax Net income (loss) Business Acquisition, Pro Forma Net Income (Loss) Earnings (loss) per share, Basic (usd per share) Business Acquisition, Pro Forma Earnings Per Share, Basic Earnings (loss) per share, Diluted (usd per share) Business Acquisition, Pro Forma Earnings Per Share, Diluted Estimated combined federal and state income tax rate (percent) Business Acquisition - Pro Forma Combined Federal and State income Tax Rate Business Acquisition - Pro Forma Combined Federal and State income Tax Rate Noncontrolling Interest [Abstract] Noncontrolling Interest [Abstract] Sellers retention (percent) Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners Payments for Repurchase of Redeemable Noncontrolling Interest Payments for Repurchase of Redeemable Noncontrolling Interest Schedule of Change in Asset Retirement Obligation Schedule of Change in Asset Retirement Obligation [Table Text Block] 2019 Defined Benefit Plan, Expected Future Benefit Payment, Next Twelve Months 2020 Defined Benefit Plan, Expected Future Benefit Payment, Year Two 2021 Defined Benefit Plan, Expected Future Benefit Payment, Year Three 2022 Defined Benefit Plan, Expected Future Benefit Payment, Year Four 2023 Defined Benefit Plan, Expected Future Benefit Payment, Year Five 2024-2028 Defined Benefit Plan, Expected Future Benefit Payment, Five Fiscal Years Thereafter Jointly Owned Facilities Jointly Owned Facilities, Utility and Non-Regulated [Text Block] Disclosure of proportionate interests in jointly owned facilities (regulated utiliyt and non-regulated). Includes the amount of plants in service, the accumulated depreciation (if available), the amount of construction in progress, and the proportionate share. Includes statements that the amounts represent the company's share in each joint facility and that it must provide its own financing. States that the company's share of direct expenses of the joint facilities is included in the corresponding operating expenses on its income statement (for example, fuel, maintenance, other operating expense). [Text Block] Oil and Gas Exploration and Production Industries Disclosures [Abstract] Statement [Table] Statement [Table] Discontinued Operations, Held-for-sale or Disposed of by Sale Discontinued Operations, Held-for-sale or Disposed of by Sale [Member] Statement [Line Items] Statement [Line Items] Future cash inflows Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Cash Inflows Future production costs Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Production Costs Future development costs, including plugging and abandonment Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Development Costs Future net cash flows Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Net Cash Flows 10% annual discount for estimated timing of cash flows Future Net Cash Flows Relating to Proved Oil and Gas Reserves, Ten Percent Annual Discount for Estimated Timing of Cash Flows Standardized measure of discounted future net cash flows Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves Oil and Gas Reserves (Unaudited) Oil and Gas Exploration and Production Industries Disclosures [Text Block] Schedule of Guarantor Obligations Schedule of Guarantor Obligations [Table Text Block] Regulated Operations [Abstract] Schedule of TCJA (Federal Tax Reform) Benefits Passed On To Customers By State Schedule of TCJA (Federal Tax Reform) Benefits Passed On To Customers By State [Table Text Block] [Table Text Block] for Schedule of TCJA (Federal Tax Reform) Benefits Passed On To Customers By State [Table] Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Statement of Comprehensive Income [Abstract] Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest Rate Swap Interest Rate Swap [Member] Commodity Contract Commodity Contract [Member] 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 Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member] Accumulated Other Comprehensive Income (Loss) AOCI 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] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss) [Line Items] Accumulated Other Comprehensive Income (Loss), Net of Tax, Period Start Accumulated Other Comprehensive Income (Loss), Net of Tax before reclassifications Other Comprehensive Income (Loss), before Reclassifications, Net of Tax Amounts reclassified from AOCI Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax Reclassification from Legal Entity Restructuring Reclassification from Legal Entity Restructuring Reclassification from Legal Entity Restructuring Reclassification of certain tax effects from AOCI Reclassification from AOCI, Current Period, Tax Accumulated Other Comprehensive Income (Loss), Net of Tax, Period End Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] Senior Unsecured Notes Due 2019 Senior Unsecured Notes Due 2019 [Member] Senior Unsecured Notes Due 2019 Debt Instrument [Line Items] Debt Instrument [Line Items] Proceeds from Sale of Interest in Corporate Unit Proceeds from Convertible Debt Equity Unit Stated Amount (usd per share) Debt Instrument, Convertible, If Applicable Market Value Greater Than Reference Price, Amount Used To Calculate Number of Shares, Numerator Debt Instrument, Convertible, If Applicable Market Value Greater Than Reference Price, Amount Used To Calculate Number of Shares, Numerator Corporate Units Ownership Interest Percentage In Subordinated Notes Corporate Units Ownership Interest Percentage In Subordinated Notes Corporate Units Ownership Interest Percentage In Subordinated Notes Debt Instrument, Subordinated Notes, Stated Principal Amount Debt Instrument, Convertible Conversion Ratio Basis, Stated Principal Amount The base principal amount of debt on which the conversion ratio is calculated. Debt Instrument, Convertible, Threshold Consecutive Trading Days Debt Instrument, Convertible, Consecutive Trading Days for Calculation of Applicable Market Value Debt Instrument, Convertible, Consecutive Trading Days for Calculation of Applicable Market Value Debt Instrument, Convertible, Conversion Ratio Debt Instrument, Convertible, Conversion Ratio Sale of Stock, Consideration Received on Transaction Sale of Stock, Consideration Received on Transaction Sale of Stock, Number of Shares Issued in Transaction Sale of Stock, Number of Shares Issued in Transaction Extinguishment of Debt, Amount Extinguishment of Debt, Amount Leases, Operating [Abstract] Operating Leases Leases of Lessee Disclosure [Text Block] Fair Value Disclosures [Abstract] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [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] Derivative Instrument [Axis] Derivative Contract [Domain] Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Fair Value, Measurements, Fair Value Hierarchy [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] Fair Value Estimate of Fair Value Measurement [Member] 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, Fair Value Disclosure 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 Federal Statutory Rate Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent State income tax (net of federal tax effect) Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent Percentage depletion Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depletion, Percent Non-controlling interest Effective Income Tax Rate Reconciliation, Noncontrolling Interest Income (Loss), Percent Equity AFUDC Effective Income Tax Rate Reconciliation Allowance For Other Funds Used During Construction, Equity The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate that can be explained by the equity portion of allowance for other funds used during construction, a nondeductible expense. Tax credits Effective Income Tax Rate Reconciliation, Tax Credit, Percent Transaction costs Effective Income Tax Rate Reconciliation, Nondeductible Expense, Restructuring Charges, Percent Accounting for uncertain tax positions adjustment Effective Income Tax Rate Reconciliation, Tax Contingency, Percent Flow-through adjustments Effective Income Tax Rate Reconciliation, Flow Through Accounting The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate that can be explained by Flow Through Accounting. Jurisdictional simplification project Effective Income Tax Reconciliation due to Restructuring Effective Income Tax Reconciliation due to Restructuring Other tax differences Effective Income Tax Rate Reconciliation, Other Adjustments, Percent IRC 172(f) carryback claim Effective Income Tax Reconciliation Net Operating Loss Carryback Effective Income Tax Reconciliation Net Operating Loss Carryback Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent Effective Income Tax Rate, Continuing Operations Effective Income Tax Rate Reconciliation, Percent Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount Investment Type [Axis] Investment Type [Axis] Investments [Domain] Investments [Domain] Fair Value Hierarchy and NAV [Domain] Common Collective Trust - Cash and Cash Equivalents Common Collective Trust, Cash And Cash Equivalents [Member] Common Collective Trust, Cash And Cash Equivalents [Member] Common Collective Trust - Equity Common Collective Trust, Equity Fund [Member] Common Collective Trust, Equity Fund [Member] Common Collective Trust - Fixed Income Common Collective Trust, Fixed Income Fund [Member] Common Collective Trued, Fixed Income Fund [Member] Common Collective Trust - Real Estate Real Estate Funds [Member] Intermediate - Term Bond Municipal Bonds [Member] Defined Benefit Plan, Plan Assets, Amount Defined Benefit Plan, Plan Assets, Amount Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments Defined Benefit Plan, Fair Value of Plan Assets, Excluding Net Asset Value investments Alternative Investment, Fair Value Disclosure Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets Defined Benefit Plan, Alternative Investments, Fair Value Of Plan Assets Percentage Of Monthly Redemption Percentage Of Monthly Redemption Percentage of shares that may be redeemed at the end of each month. Percentage Of Quarterly Redemption Percentage Of Quarterly Redemption Percentage of redemptions available at the end of each quarter. Investment Redemption, Notice Period Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Investment Redemption, Notice Period Unproved oil and gas properties Capitalized Costs, Unproved Properties Proved oil and gas properties Capitalized Costs, Proved Properties Gross capitalized costs Capitalized Costs, Oil and Gas Producing Activities, Gross Accumulated depreciation, depletion and amortization and valuation allowances Capitalized Costs, Accumulated Depreciation, Depletion, Amortization and Valuation Allowance Relating to Oil and Gas Producing Activities Net capitalized costs Capitalized Costs, Oil and Gas Producing Activities, Net Guarantees Guarantees [Text Block] Proved Developed and Undeveloped Oil and Gas Reserve Quantities [Table] Consultant Name [Axis] Consultant Name [Axis] Consultant Name [Axis] Consultant Name [Domain] Consultant Name [Domain] [Domain] for Consultant Name [Axis] Cawley Gillespie & Associates - Mr. Zane Meekins Cawley Gillespie & Associates - Mr. Zane Meekins [Member] Cawley Gillespie & Associates - Mr. Zane Meekins [Member] Reserve Quantities by Type of Reserve [Axis] Petroleum Reserves [Axis] Type of Reserve [Domain] Type of Reserve [Domain] Oil Oil [Member] Natural Gas Natural Gas [Member] Natural Gas Liquids Natural Gas Liquids [Member] Oil and Gas by Location [Axis] Oil and Gas by Location [Axis] Oil and Gas by Location, basin or state [Axis] Location [Domain] Location [Domain] Piceance Basin Piceance Basin [Member] Piceance Basin - oil and gas drilling location, primarily in Colorado [Member] San Juan Basin San Juan Basin [Member] San Juan Basin [Member] Other Basin's Other Basin's [Member] Other Basin's [Member] Reserve Quantities [Line Items] Reserve Quantities [Line Items] Discounted Present Value Rate Used in Estimating Future Net Revenues - Oil and Gas Industry Discounted Present Value Rate Used in Estimating Future Net Revenues - Oil and Gas Industry Discounted Present Value Rate Used in Estimating Future Net Revenues - Oil and Gas Industry Practical Experience In Petroleum Engineering Practical Experience In Petroleum Engineering Practical Experience In Petroleum Engineering Experience In Estimation and Evaluation of Reserves Experience In Estimation and Evaluation of Reserves Experience In Estimation and Evaluation of Reserves Proved Developed And Undeveloped Reserves [Roll Forward] Proved Developed And Undevleoped Reserves [Roll Forward] Proved Developed And Undevleoped Reserves [Roll Forward] Balance at Beginning of Year Proved Developed and Undeveloped Reserves, Net Production Proved Developed and Undeveloped Reserves, Production Additions, Acquisitions (Sales) Proved Developed and Undeveloped Reserves Purchase and Sale of Minerals in Place Purchase and Sale of Minerals in Place of proved developed and undeveloped reserves Additions, Extensions, Discoveries (bcfe) Proved Developed and Undeveloped Reserves, Extensions, Discoveries, and Additions Reserves, Revisions of Previous Estimates Proved Developed and Undeveloped Reserves, Revisions of Previous Estimates Balance at End of Year Proved Developed Reserves (Volume) Proved Developed Reserves (Volume) Proved Undeveloped Reserve (Volume) Proved Undeveloped Reserve (Volume) Average Crude Oil Price, Per Barrel, NYMEX Average Crude Oil Price, Per Barrel, NYMEX Average Crude Oil Price, Per Barrel, NYMEX. Average price is calculated using the price at the first day of each month for each of the preceding 12 months. Average Natural Gas Price Per MCF, NYMEX Average Natural Gas Price Per MCF, NYMEX Average Natural Gas Price Per MCF, NYMEX. The average price is calculated using the price at the first day of each month for each of the preceding 12 months. Average Natural Gas Liquids Price Per MCF, NYMEX Average Natural Gas Liquids Price Per MCF, NYMEX Average Natural Gas Liquids Price Per MCF, NYMEX. The average price is calculated using the price at the first day of each month for each of the preceding 12 months. Average Crude Oil Price Per Barrel, Wellhead Average Crude Oil Price Per Barrel, Wellhead Average Crude Oil Price Per Barrel, Wellhead. The average price is calculated using the price at the first day of each month for each of the preceding 12 months. Average Natural Gas Price Per MCF, Wellhead Average Natural Gas Price Per MCF, Wellhead Average Natural Gas Price Per MCF, Wellhead. The average price is calculated using the price at the first day of each month for each of the preceding 12 months. Average Natural Gas Liquids Price Per MCF, Wellhead Average Natural Gas Liquids Price Per MCF, Wellhead Average Natural Gas Liquids Price Per MCF, Wellhead. The average price is calculated using the price at the first day of each month of the preceding 12 months. Reserve Calculation - Costs To Gather And Process Natural Gas Previously Netted From The Gas Price Were Reclassified Into Operating Expenses Reserve Calculation - Costs To Gather And Process Natural Gas Previously Netted From The Gas Price Were Reclassified Into Operating Expenses Reserve Calculation - Costs To Gather And Process Natural Gas Previously Netted From The Gas Price Were Reclassified Into Operating Expenses Supplemental Cash Flow Information [Abstract] Supplemental Disclosure of Cash Flow Information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Benefit plan liability adjustments - net gain (loss), Tax Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax Benefit plan liability adjustments - prior service (costs), Tax Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Tax Reclassification adjustment of benefit plan liability - net gain (loss) tax Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax Reclassification adjustment of benefit plan liability - prior service cost, tax Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax Fair value adjustment on derivatives (interest rate swaps) designated as cash flow hedges, Tax Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Tax Reclassification adjustment of cash flow hedges settled (interest rate swaps) and included in net income (loss), Tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax Derivative [Table] Derivative [Table] Energy [Axis] Energy [Axis] Energy [Domain] Energy [Domain] Natural Gas, Distribution Natural Gas, Distribution [Member] Natural Gas for distribution [Member] Future Future [Member] Commodity Option Commodity Option [Member] Basis Swap Basis Swap [Member] Fixed for Float Swaps Purchased Fixed for Float Swaps Purchased [Member] Fixed for Float Swaps Purchased [Member] Natural Gas Physical Purchases Natural Gas Physical Purchases [Member] Natural Gas Physical Purchases [Member] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Cash Flow Hedging Cash Flow Hedging [Member] Derivative [Line Items] Derivative [Line Items] Derivative, Nonmonetary Notional Amount Derivative, Nonmonetary Notional Amount Derivative, Remaining Maturity Derivative, Remaining Maturity Derivative, Nonmonetary Notional Amount, Energy Measure Derivative, Nonmonetary Notional Amount, Energy Measure Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Income Taxes Income Tax Disclosure [Text Block] Regulatory Liability [Axis] Regulatory Liability [Axis] Regulatory Liability [Domain] Regulatory Liability [Domain] Deferred Income Tax Charge Deferred Income Tax Charges a [Member] Rate Action of a regulator resulting in capitalization or accrual of income taxes Revenue Subject to Refund Revenue Subject to Refund [Member] Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Tax Cuts and Jobs Act of 2017, Incomplete Accounting, Provisional Income Tax Expense (Benefit) Increase (Decrease) in Regulatory Liabilities Increase (Decrease) in Regulatory Liabilities Revenue Refunded To Customers As A Result Of The TCJA Tax Benefits Revenue Refunded To Customers As A Result Of The TCJA Tax Benefits Revenue Refunded To Customers As A Result Of The TCJA Tax Benefits Disposal Groups, Including Discontinued Operations [Table] Disposal Groups, Including Discontinued Operations [Table] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] Disposal Group, Including Discontinued Operations, Impairment Of Long-lived Assets (Net of Tax) Disposal Group, Including Discontinued Operations, Impairment Of Long-lived Assets (Net of Tax) Disposal Group, Including Discontinued Operations, Impairment Of Long-lived Assets (Net of Tax) Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract] Disposal Group, Including Discontinued Operation, Balance Sheet Disclosures [Abstract] Other current assets Disposal Group, Including Discontinued Operation, Other Assets, Current Deferred income tax assets, noncurrent, net Disposal Group, Including Discontinued Operation, Deferred Tax Assets, Noncurrent Property, plant and equipment, net Disposal Group, Including Discontinued Operation, Property, Plant and Equipment Other current liabilities Disposal Group, Including Discontinued Operation, Other Liabilities Other noncurrent liabilities Disposal Group, Including Discontinued Operation, Other Liabilities, Noncurrent Net assets Disposal Group, Including Discontinued Operation, Assets Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] Disposal Group, Including Discontinued Operation, Income Statement Disclosures [Abstract] Revenue Disposal Group, Including Discontinued Operation, Revenue Operations and maintenance Disposal Group, Including Discontinued Operation, Operating Expense Loss on sale of assets Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax Depreciation, depletion and amortization Disposal Group, Including Discontinued Operation, Depreciation and Amortization Impairment of long-lived assets Disposal Group, Including Discontinued Operations, Impairment of Long-lived Assets Disposal Group, Including Discontinued Operations, Impairment of Long-lived Assets Total operating expenses Disposal Group, Including Discontinued Operations, Total Operating Expenses Disposal Group, Including Discontinued Operations, Total Operating Expenses Operating (loss) Disposal Group, Including Discontinued Operation, Operating Income (Loss) Interest income (expense), net Disposal Group, Including Discontinued Operations, Interest Income (Expense), Net Disposal Group, Including Discontinued Operations, Interest Income (Expense), Net Other income (expense), net Disposal Group, Including Discontinued Operations, Other Income (Expense), net Disposal Group, Including Discontinued Operations, Other Income (Expense), net Income tax benefit Discontinued Operation, Tax Effect of Discontinued Operation (Loss) from discontinued operations Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent Public Utilities General Disclosures [Table] Public Utilities General Disclosures [Table] Restatement [Axis] Restatement [Axis] Restatement [Domain] Restatement [Domain] Previously Reported Previously Reported [Member] Other regulatory liabilities Other Regulatory Liabilities [Member] Other regulatory liabilities [Member] Corriedale Wind Project Corriedale Wind Project [Member] Corriedale Wind Project [Member] Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Assets [Domain] Regulatory Asset [Domain] Environmental Restoration Costs Environmental Restoration Costs [Member] Other regulatory assets Other Regulatory Assets [Member] Other regulatory assets [Member] 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] Federal Energy Regulatory Commission (FERC) Common Use System (CUS) Federal Energy Regulatory Commission (FERC) Common Use System (CUS) [Member] Federal Energy Regulatory Commission (FERC) Common Use System (CUS) [Member] South Dakota Public Utilities Commission (SDPUC) South Dakota Public Utilities Commission (SDPUC) [Member] South Dakota Public Utilities Commission (SDPUC) [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] South Dakota Electric and Wyoming Electric South Dakota Electric and Wyoming Electric [Member] South Dakota Electric and Wyoming Electric [Member] Black Hills Energy, Wyoming Electric Black Hills Energy, Wyoming Electric [Member] Black Hills Energy, Wyoming Electric [Member] South Dakota Electric South Dakota Electric [Member] South Dakota Electric [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 Public Utilities, Aggregate Amount of Customer Credits Through The Power Cost Adjustment Mechanism Public Utilities, Aggregate Amount of Customer Credits Through The Power Cost Adjustment Mechanism Public Utilities, Aggregate Amount of Customer Credits Through The Power Cost Adjustment Mechanism Public Utilities, Power Purchase Agreement Annual Cost Escalation Percentage Through 2022 Public Utilities, Power Purchase Agreement Annual Cost Escalation Percentage Through 2022 Public Utilities, Power Purchase Agreement Annual Cost Escalation Percentage Through 2022 Regulatory liabilities Regulatory Liabilities Annual Revenue Requirement, as Required by the FERC Joint-Access Transmission Tariff Annual Revenue Requirement, as Required by the FERC Joint-Access Transmission Tariff Annual Revenue Requirement, as Required by the FERC Joint-Access Transmission Tariff Capital Addition Requirements by the FERC Joint-Access Transmission Tariff Capital Addition Requirements by the FERC Joint-Access Transmission Tariff Capital Addition Requirements by the FERC Joint-Access Transmission Tariff Public Utilities, Moratorium Period Public Utilities, Moratorium Period Public Utilities, Moratorium Period. Period that rates can not be raised, absent an extraordinary event. Public Utilities, Suspension Of Rate Increase Due to a Moratorium by State Regulators Public Utilities, Suspension Of Rate Increase Due to a Moratorium by State Regulators Public Utilities, Suspension Of Rate Increase Due to a Moratorium by State Regulators Regulatory assets Regulatory Assets Regulatory Asset, Amortization Period Regulatory Asset, Amortization Period Public Utilities, Increase in Amortization Expense Due to Change in Amortization Periods Public Utilities, Increase in Amortization Expense Due to Change in Amortization Periods Public Utilities, Increase in Amortization Expense Due to Change in Amortization Periods Public Utilities, Approved Rate Increase (Decrease), Amount Public Utilities, Approved Rate Increase (Decrease), Amount Tax Credit Carryforward [Table] Tax Credit Carryforward [Table] Valuation Allowance by Deferred Tax Asset [Axis] Valuation Allowance by Deferred Tax Asset [Axis] Deferred Tax Asset [Domain] Deferred Tax Asset [Domain] Valuation Allowance, Operating Loss Carryforwards SEC Schedule, 12-09, Valuation Allowance, Operating Loss Carryforward [Member] Tax Credit Carryforward [Axis] Tax Credit Carryforward [Axis] Tax Credit Carryforward, Name [Domain] Tax Credit Carryforward, Name [Domain] Investment Tax Credit Carryforward Investment Tax Credit Carryforward [Member] Research Tax Credit Carryforward Research Tax Credit Carryforward [Member] Deferred Tax Asset State Tax Credit [Member] State Tax Credit [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 Tax Credit Carryforward [Line Items] Tax Credit Carryforward [Line Items] Deferred Tax Assets, State Tax Credits Deferred Tax Assets, State Taxes Tax Credit Carryforward, Valuation Allowance Tax Credit Carryforward, Valuation Allowance Valuation Allowance, Deferred Tax Asset, Change in Amount Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount Income Tax Expense (Benefit) Income Tax Expense (Benefit) Income Statement [Abstract] Revenue: Revenues [Abstract] Revenue 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. 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. Depreciation, depletion and amortization Depreciation, Depletion and Amortization Taxes - property and production Taxes, Miscellaneous Other operating expenses Other Cost and Expense, Operating Total operating expenses Operating Expenses Operating income Operating Income (Loss) 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) Total Other Income Expense Total Other Income Expense Income before income taxes Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest Income tax benefit (expense) Income from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest Net (loss) from discontinued operations Net income Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Net income attributable to noncontrolling interest Net Income (Loss) Attributable to Noncontrolling Interest Net income (loss) available for common stock Amounts attributable to common shareholders: Income Amounts Attributable to Parent, Disclosures [Abstract] Net income from continuing operations Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent Earnings (loss) per share of common stock, Basic - Earnings Per Share, Basic [Abstract] Earnings from continuing operations, Basic (usd per share) Income (Loss) from Continuing Operations, Per Basic Share (Loss) from discontinued operations per share, Basic (usd per share) Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share Total earnings (loss) per share of common stock, Basic (usd per share) Earnings Per Share, Basic Earnings (loss) per share of common stock, Diluted - Earnings Per Share, Diluted [Abstract] Earnings 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 Total earnings (loss) per share of common stock, Diluted (usd per share) Earnings Per Share, Diluted Weighted average common shares outstanding: Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract] Weighted average number of shares of common stock, basic and diluted[Abstract] Basic Weighted Average Number of Shares Outstanding, Basic Diluted Weighted Average Number of Shares Outstanding, Diluted Quarterly Financial Information Disclosure [Abstract] Selected Quarterly Financial Information [Table] Selected Quarterly Financial Information [Table] Selected Quarterly Financial Information [Table] Restructuring Type [Axis] Restructuring Type [Axis] Type of Restructuring [Domain] Type of Restructuring [Domain] Other Restructuring Other Restructuring [Member] Nature of Tax Credit [Axis] Nature of Tax Credit [Axis] Nature of Tax Credit [Axis] Nature of Tax Credit [Domain] Nature of Tax Credit [Domain] [Domain] for Nature of Tax Credit [Axis] True-up from SourceGas Tax Returns True-up from SourceGas Tax Returns [Member] True-up from SourceGas Tax Returns [Member] Selected Quarterly Financial Information [Line Items] Selected Quarterly Financial Information [Line Items] [Line Items] for Selected Quarterly Financial Information [Table] Operating income (loss) Income (loss) from continuing operations Total earnings (loss) per share of common stock, Basic (usd per share) Total earnings (loss) per share of common stock, Diluted (usd per share) Deferred Income Tax Expense (Benefit) Deferred Income Tax Expense (Benefit) Business Combination, Acquisition Related Costs, Net Of Tax Business Combination, Acquisition Related Costs, Net Of Tax Business Combination, Acquisition Related Costs, Net Of Tax - This element represents the tax effected Acquisition-related costs incurred to effect the business combination which costs have been expensed during the period. Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Income Tax Expense (Benefit) Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Income Tax Expense (Benefit) Tax Cuts and Jobs Act of 2017, Change in Tax Rate, Income Tax Expense (Benefit) Income Tax Expense (Benefit) Impairment Of Oil And Gas Properties, Net Of Tax Impairment Of Oil And Gas Properties, Net Of Tax Impairment Of Oil And Gas Properties, Net Of Tax Restricted Stock Restricted Stock [Member] Award vesting period Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested Number of Shares [Roll Forward] Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward] Restricted Stock balance at beginning of period Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number Shares Granted Shares Vested Shares Forfeited Restricted Stock at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract] Restricted Stock and RSUs, total fair value of shares vested Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value Investment [Table] Investment [Table] Asset Class [Axis] Asset Class [Axis] Asset Class [Domain] Asset Class [Domain] Cost-method Investments Cost-method Investments [Member] Cash Surrender Value Cash Surrender Value [Member] Investment [Line Items] Investment [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 Investments Long-term Investments Supply Commitment [Axis] Supply Commitment [Axis] Supply Commitment Arrangement [Domain] Supply Commitment Arrangement [Domain] M D U, Montana Dakota Utilities M D U, Montana Dakota Utilities [Member] MDU, Montana Dakota Utilities - utility that is an undivided owner of Wygen III[Member] City Of Gillette City Of Gillette [Member] City Of Gillette, Wyoming, an undivided owner in Wygen III [Member] Purchase Power Contract, MEAN, 10 Megawatts Purchase Power Contract, MEAN, 10 Megawatts [Member] Long-ter Purchase Power Contract with MEAN for 10 megawatts [Member] Macquarie Energy, LLC Supply Agreement Macquarie Energy, LLC Supply Agreement [Member] Macquarie Energy, LLC Supply Agreement [Member] Neil Simpson I I Neil Simpson I I [Member] Neil Simpson II power generation faciltiy located near Gillette WY [Member] Megawatts Sold Under Long-Term Contract Number of Megawatts Sold Under Long-Term Contract Number of Megawatts Sold Under Long-Term Contract Long-term Contract To Sell Electric Power, Date of Contract Expiration Long-term Contract To Sell Electric Power, Date of Contract Expiration Date of contract expiration of capacity and energy arrangements in which the entity must commit resources to supply goods or services to one or more customers. Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Axis] Property, Plant and Equipment, Type [Domain] Property, Plant and Equipment, Type [Domain] Assets Not Expected To Be Utilized In Cost Of Service Gas Program Assets Not Expected To Be Utilized In Cost Of Service Gas Program [Member] Assets Not Expected To Be Utilized In Cost Of Service Gas Program [Member] Results of Operations, Impairment of Oil and Gas Properties Results of Operations, Impairment of Oil and Gas Properties Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Net (loss) from Discontinued Operations Net (loss) from Discontinued Operations [Member] Net (loss) from Discontinued Operations [Member] Fuel, purchased power and cost of natural gas sold Cost of Sales [Member] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Not Designated as Hedging Instrument Not Designated as Hedging Instrument [Member] Deferred Derivative Gain (Loss) Deferred Derivative Gain (Loss) [Member] Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Derivative, Gain (Loss) on Derivative, Net Derivative, Gain (Loss) on Derivative, Net Property, Plant and Equipment [Abstract] Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] Purchase Option, Property Purchase Option, Property [Member] A contract between two parties, the buyer and the seller of the option, where the buyer has the right but not the obligation to buy a particular property, plant and equipment from the seller of the option at a certain time (the expiration date) for a certain price (the strike price). [Member] Long-term Contract for Purchase of Electric Power, Date of Contract Expiration Long-term Contract for Purchase of Electric Power, Date of Contract Expiration Asset Purchase Option Asset Purchase Option The original fixed option amount per Megawatt to be paid to purchase property, plant and equipment in the future prior to any adjustments. Property, Plant and Equipment, Useful Life Property, Plant and Equipment, Useful Life Arkansas Public Service Commission (APSC) Arkansas Public Service Commission (APSC) [Member] Arkansas Public Service Commission (APSC) Colorado Public Utilities Commission (CPUC) Colorado Public Utilities Commission (CPUC) [Member] Colorado Public Utilities Commission (CPUC) [Member] Iowa Utilities Board (IUB) Iowa Utilities Board (IUB) [Member] Iowa Utilities Board (IUB) [Member] Kansas Public Utilities Commission (KPUC) Kansas Public Utilities Commission (KPUC) [Member] Kansas Public Utilities Commission (KPUC) [Member] Nebraska Public Service Commission (NPSC) Nebraska Public Service Commission (NPSC) [Member] Nebraska Public Service Commission (NPSC) [Member] Number of States That Have Received State Utility Commission Approvals to Provide the Benefits of Federal Tax Reform to Utility Customers Number of States That Have Received State Utility Commission Approvals to Provide the Benefits of Federal Tax Reform to Utility Customers Number of States That Have Received State Utility Commission Approvals to Provide the Benefits of Federal Tax Reform to Utility Customers Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount Public Utilities, Number of Rate Review Requests Filed Public Utilities, Number of Rate Review Requests Filed Public Utilities, Number of Rate Review Requests Filed Business Segment Information Segment Reporting Disclosure [Text Block] Disposal Groups, Including Discontinued Operations, Balance Sheet Accounts Disposal Groups, Including Discontinued Operations, Balance Sheet Accounts [Table Text Block] Disposal Groups, Including Discontinued Operations, Balance Sheet Accounts [Table Text Block] Disposal Groups, Including Discontinued Operations, Income Statement Disposal Groups, Including Discontinued Operations, Income Statement [Table Text Block] Disposal Groups, Including Discontinued Operations, Income Statement [Table Text Block] Revenue Results of Operations, Revenue from Oil and Gas Producing Activities Production costs Results of Operations, Production or Lifting Costs Depreciation, depletion and amortization Results of Operations, Depreciation, Depletion and Amortization, and Valuation Provisions Impairment of long-lived assets Total costs Results of Operations, Expense from Oil and Gas Producing Activities Results of operations from producing activities before tax Results of Operations, Income before Income Taxes Income tax benefit (expense) Results of Operations, Income Tax Expense Results of operations from producing activities (excluding general and administrative costs and interest costs) Results of Operations, Oil and Gas Producing Activities Net Income (Excluding Corporate Overhead and Interest Costs) Interest rate swaps Commodity derivatives Net income (loss) Other comprehensive income (loss), net of tax: Other Comprehensive Income (Loss), Net of Tax [Abstract] Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax [Abstract] Other Comprehensive (Income) Loss, Defined Benefit Plan, before Reclassification Adjustment, after Tax [Abstract] Benefit plan liability adjustments - net gain (loss) (net of tax of $(660), $1,030 and $757, respectively) Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, after Tax Benefit plan liability adjustments - prior service (costs) (net of tax of $0, $0 and $107, respectively) Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), after Tax Reclassification adjustment of benefit plan liability - net gain (loss) (net of tax of $(586), $(585) and $(600), respectively) Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, after Tax Reclassification adjustment of benefit plan liability - prior service cost (net of tax of $43, $69 and $67, respectively) Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract] Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), after Reclassification and Tax [Abstract] Net unrealized gains (losses) net of tax Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, Net of Tax Reclassification of net realized (gains) losses on settled/amortized derivatives, net of tax Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax Other comprehensive income (loss), net of tax Other Comprehensive Income (Loss), Net of Tax Comprehensive income Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest Comprehensive income available for common stock Comprehensive Income (Loss), Net of Tax, Attributable to Parent Black Hills Corporation Black Hills Corporation [Member] Black Hills Corporation [Member] Wyoming Electric Wyoming Electric [Member] Wyoming Electric Variable Rate [Axis] Variable Rate [Axis] Variable Rate [Domain] Variable Rate [Domain] London Interbank Offered Rate (LIBOR) London Interbank Offered Rate (LIBOR) [Member] Senior Unsecured Notes Due 2023 Senior Unsecured Notes Due 2023 [Member] Senior Unsecured Notes Due 2023 [Member] Senior Unsecured Notes Due 2020 Senior Unsecured Notes Due 2020 [Member] Senior Unsecured Notes Due 2020 [Member] Senior Unsecured Notes Due 2026 Senior Unsecured Notes Due 2026 [Member] Senior Unsecured Notes Due 2026 [Member] Senior Unsecured Notes Due 2027 Senior Unsecured Notes Due 2027 [Member] Senior Unsecured Notes Due 2027 [Member] Senior Unsecured Notes Due 2033 Senior Unsecured Notes Due 2033 [Member] Senior Unsecured Notes Due 2033 [Member] Senior Unsecured Notes Due 2046 Senior Unsecured Notes Due 2046 [Member] Senior Unsecured Notes Due 2046 [Member] Corporate Term Loan Due August 2019 Corporate Term Loan Due August 2019 [Member] Corporate Term Loan Due August 2019 [Member] Corporate Term Loan Due July 2020 Corporate Term Loan Due July 2020 [Member] Corporate Term Loan Due July 2020 [Member] Corporate Term Loan Due June 2021 Corporate Term Loan Due June 2021 [Member] Corporate Term Loan Due June 2021 [Member] First Mortgage Bonds Due 2044 First Mortgage Bonds Due 2044 [Member] First Mortgage Bonds Due 2044 [Member] First Mortgage Bonds Due 2032 First Mortgage Bonds Due 2032 [Member] First Mortgage Bonds Due 2032 [Member] First Mortgage Bonds Due 2039 First Mortgage Bonds Due 2039 [Member] First Mortgage Bonds Due 2039 [Member] First Mortgage Bonds Due 2037 First Mortgage Bonds Due 2037 [Member] First Mortgage Bonds Due 2037 [Member] Industrial Development Revenue Bonds Due 2021 Industrial Development Revenue Bonds Due 2021 [Member] Industrial Development Revenue Bonds Due 2021 [Member] Industrial Development Revenue Bonds Due 2027 Industrial Development Revenue Bonds Due 2027 [Member] Industrial Development Revenue Bonds Due 2027 [Member] Series 94 A Debt, Due 2024 Bonds Due 2024 [Member] Bonds Due 2024 [Member] Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility Revolving Credit Facility [Member] Interest rate (percent) Debt Instrument, Interest Rate, Stated Percentage Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate Long-term Debt, Percentage Bearing Variable Interest, Percentage Rate Long-term debt Long-term Debt, Gross Less unamortized debt discount Debt Instrument, Unamortized Discount Total long-term debt Long Term Debt Before The Deduction Of Deferred Financing Costs Amount of long term debt before the deduction of deferred financing costs, but after the deduction of unamortized (discount) premium and includes current maturities. Less current maturities Long-term Debt, Current Maturities Less unamortized deferred financing costs Debt Issuance Costs, Net Long-term debt, net of current maturities Long-term Debt, Excluding Current Maturities Cost Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities Disclosure Cost Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities Disclosure [Table Text Block] Schedule of Proved Developed and Undeveloped Oil and Gas Reserve Quantities Schedule of Proved Developed and Undeveloped Oil and Gas Reserve Quantities [Table Text Block] Capitalized Costs Relating to Oil and Gas Producing Activities Disclosure Capitalized Costs Relating to Oil and Gas Producing Activities Disclosure [Table Text Block] Results of Operations for Oil and Gas Producing Activities Disclosure Results of Operations for Oil and Gas Producing Activities Disclosure [Table Text Block] Schedule of Capitalized Costs of Unproved Properties Excluded from Amortization Schedule of Capitalized Costs of Unproved Properties Excluded from Amortization [Table Text Block] Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserves Disclosure [Table Text Block] Changes In Standardized Measure of Discounted Future Cash Flows Relating to Proved Reserve Disclosures Schedule of Changes in Standardized Measure of Discounted Future Net Cash Flows [Table Text Block] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Property, plant and equipment Property, Plant and Equipment, Gross Accounts Receivable and Allowance for Doubtful Accounts Schedule of Accounts, Notes, Loans and Financing Receivable [Table Text Block] Schedule of Valuation and Qualifying Accounts Disclosure SEC Schedule, 12-09, Schedule of Valuation and Qualifying Accounts Disclosure [Text Block] Disaggregation of Revenue Disaggregation of Revenue [Table Text Block] Materials, Supplies and Fuel Schedule of Inventory, Current [Table Text Block] Investments Investment [Table Text Block] Schedule of Accrued Liabilities Schedule of Accrued Liabilities [Table Text Block] Goodwill Schedule of Goodwill [Table Text Block] Intangible Assets Schedule of Finite-Lived Intangible Assets [Table Text Block] 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] Earnings Per Share of Common Stock Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Antidilutive Securities Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block] Employee Benefit Plans Pension and Other Postretirement Benefits Disclosure [Text Block] Fair Value, by Balance Sheet Grouping [Table] Fair Value, by Balance Sheet Grouping [Table] Carrying Amount Reported Value Measurement [Member] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] Cash and cash equivalents - carrying amount Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents, Fair Value Disclosure Cash and Cash Equivalents, Fair Value Disclosure Restricted cash - carrying amount Restricted Cash and Investments, Current Restricted Cash Fair Value Disclosure Restricted Cash Fair Value Disclosure This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This element represents the fair value of assets categorized as restricted cash Notes payable Short-term Debt Notes payable - fair value Short-term Debt, Fair Value Long-term debt, including current maturities - carrying amount Long-term Debt Long-term debt, including current maturities - fair value Long-term Debt, Fair Value Supplemental Disclosure of Cash Flow Information Cash Flow, Supplemental Disclosures [Text Block] Base Rate Base Rate [Member] Eurodollar Eurodollar [Member] Extinguishment of Debt [Axis] Extinguishment of Debt [Axis] Extinguishment of Debt, Type [Domain] Extinguishment of Debt, Type [Domain] Junior Subordinated Debt Junior Subordinated Debt [Member] Corporate Term Loan Due 2033 Corporate Term Loan Due 2033 [Member] Corporate Term Loan Due 2033 Proceeds from senior unsecured notes Proceeds from Issuance of Senior Long-term Debt Business Description and Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] 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] Sharing Arrangement with the City of Gillette, Wyoming Sharing Arrangement with the City of Gillette, Wyoming [Member] Sharing Arrangement with the City of Gillette, Wyoming [Member] PacifiCorp Purchase Power Agreement PacifiCorp Purchase Power Agreement [Member] PacifiCorp PPA [Member] PacifiCorp Transmission PacifiCorp Transmission [Member] PacifiCorp Transmission [Member] Happy Jack Wind Purchase Power Agreement Happy Jack Wind Purchase Power Agreeement [Member] Happy Jack Wind Purchase Power Agreeement [Member] Silver Sage Wind Power Purchase Agreement Silver Sage Wind Power Purchase Agreement [Member] Silver Sage Wind Power Purchase Agreement [Member] Platte River Power Authority Wind Power Agreement Platte River Power Authority Wind Power Agreement [Member] Platte River Power Authority Wind Power Agreement [Member] Cargill Power Purchase Agreement Cargill Power Purchase Agreement [Member] Cargill Power Purchase Agreement [Member] Subsidiary of Common Parent Subsidiary of Common Parent [Member] Renewable Wind Energy, Wyoming Electric Renewable Wind Energy, Wyoming Electric [Member] Renewable wind energy from Wyoming Electric AltaGas AltaGas [Member] AltaGas Black Hills Electric Generation and Colorado Electric Black Hills Electric Generation and Colorado Electric [Member] Black Hills Electric Generation and Colorado Electric [Member] Number of Megawatts Capacity Sold Number of Megawatts Capacity Sold Number of Megawatts Capacity Sold Long-term Purchase Commitment, Period Long-term Purchase Commitment, Period Megawatts of Capacity Purchased Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Long-term Contract for Purchase of Electric Power, Share of Plant Output Being Purchased Cost of Purchased Power Cost of Goods and Services Sold Business Acquisition, Percentage of Voting Interests Acquired Business Acquisition, Percentage of Voting Interests Acquired Jointly Owned Utility Plant, Proportionate Ownership Share Schedule of Long-term Debt Instruments Schedule of Long-term Debt Instruments [Table Text Block] Schedule of Maturities of Long-term Debt Schedule of Maturities of Long-term Debt [Table Text Block] Deferred Financing Costs Deferred Financing Costs [Table Text Block] Schedule of Deferred Financing Costs expensed on the Statement of Income and/or net amount presented on the Statement of Financial Position [Table Text Block] Natural Gas Indices [Axis] Natural Gas Indices [Axis] Natural Gas Indices [Axis] Natural Gas Indices [Domain] Natural Gas Indices [Domain] [Domain] for Natural Gas Indices [Axis] CIG Rockies CIG Rockies [Member] Colorado Interstate Gas - Rockies [Member]