Organization, Consolidation and Presentation of Financial Statements [Abstract] Accrued employee compensation, benefits and withholdings Employee-related Liabilities, Current Accrued property taxes Accrual for Taxes Other than Income Taxes, Current Accrued payments related to litigation expenses and settlements Loss Contingency, Accrual, Current Customer deposits and prepayments Customer Advances and Deposits, Current Accrued interest and contract adjustment payments Interest Payable, Current Other (none of which is individually significant) Other Accrued Liabilities, Current Total accrued liabilities Accrued Liabilities, Current Fair Value Disclosures [Abstract] Fair Value Measurements Derivatives and Fair Value [Text Block] Schedule of Jointly Owned Utility Plants Schedule Of Jointly Owned Facilities [Table Text Block] Tabular disclosure of proportionate interests in jointly owned plants. Includes the amount of plants in service, the accumulated provision for depreciation (if available), the amount of facilities under construction, and the proportionate share. Includes statements that the amounts represent the company's share in each joint plant and that it must provide its own financing. Information concerning two or more plants on the same site may be combined. 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, maintenance and other operating expense). This schedule includes both Utility and non-Utility facilities. [Table Text Block] Income Tax Disclosure [Abstract] Income Taxes Income Tax Disclosure [Text Block] Stockholders' Equity Note [Abstract] Schedule of Long-term Debt Instruments [Table] Schedule of Long-term Debt Instruments [Table] 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] Range [Axis] Range [Axis] Range [Domain] Range [Domain] Minimum Minimum [Member] Maximum Maximum [Member] Legal Entity [Axis] Legal Entity [Axis] Entity [Domain] Entity [Domain] Black Hills Corporation Black Hills Corporation [Member] Black Hills Corporation [Member] Debt Instrument [Line Items] Debt Instrument [Line Items] Issuance of equity units Debt Instrument, Convertible, Number of Equity Instruments 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 Price (usd per share) Debt Instrument, Convertible, Conversion Price Debt Instrument, Convertible, Conversion Ratio Debt Instrument, Convertible, Conversion Ratio Debt Instrument, Convertible, Reference Price (usd per share) Debt Instrument, Convertible, Reference Price Debt Instrument, Convertible, Reference Price RSN Interest Rate (percent) Debt Instrument, Interest Rate, Stated Percentage Stock Purchase Contract Rate Equity Unit, Dividend Rate, Percentage, Annual Equity Unit, Dividend Rate, Percentage (annual) Equity Unit, Annual Dividend Amount (usd per share) Equity Unit, Annual Dividend Amount Equity Unit, Annual Dividend Amount Premium on Publicly-Traded Equity Units Known As Corporate Units Premium on Publicly-Traded Equity Units Known As Corporate Units Premium on Publicly-Traded Equity Units Known As Corporate Units Equity units - issuance Proceeds from Sale of Interest in Corporate Unit Long-term debt Long-term Debt, Gross Stock Purchase Contract Liability Debt Instrument, Convertible, Present Value Of Future Contract Adjustment Payments Debt Instrument, Convertible, Present Value Of Future Contract Adjustment Payments Business Combinations [Abstract] Schedule of Business Acquisitions, by Acquisition [Table] Schedule of Business Acquisitions, by Acquisition [Table] Segments [Axis] Segments [Axis] Segments [Domain] Segments [Domain] Gas Utilities Gas Utilities [Member] Gas Utilities [Member] Business Acquisition [Axis] Business Acquisition [Axis] Business Acquisition, Acquiree [Domain] Business Acquisition, Acquiree [Domain] Source Gas Source Gas [Member] Source Gas [Member] SourceGas Transaction SourceGas Transaction [Member] SourceGas Transaction [Member] Equity Components [Axis] Equity Components [Axis] Equity Component [Domain] Equity Component [Domain] Common Stock Common Stock [Member] Consolidation Items [Axis] Consolidation Items [Axis] Consolidation Items [Domain] Consolidation Items [Domain] Corporate, Non-Segment Corporate, Non-Segment [Member] Black Hills Energy, Arkansas Black Hills Energy, Arkansas [Member] Black Hills Energy, Arkansas [Member] Rocky Mountain Natural Gas Rocky Mountain Natural Gas [Member] Rocky Mountain Natural Gas [Member] Black Hills Gas Distribution, Colorado Black Hills Gas Distribution, Colorado [Member] Black Hills Gas Distribution, Colorado [Member] Black Hills Gas Distribution - Nebraska Black Hills Gas Distribution - Nebraska [Member] Black Hills Gas Distribution - Nebraska [Member] Black Hills Gas Distribution - Wyoming Black Hills Gas Distribution - Wyoming [Member] Black Hills Gas Distribution - Wyoming Regulatory Asset [Axis] Regulatory Asset [Axis] Regulatory Assets [Domain] Regulatory Asset [Domain] Gas Supply Contract Termination Gas Supply Contract Termination [Member] Gas Supply Contract Termination [Member] Business Acquisition [Line Items] Business Acquisition [Line Items] Acquisition Narrative [Abstract] Acquisition Narrative [Abstract] Acquisition Narrative [Abstract] 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 Long-term debt - issuance Proceeds from Issuance of Long-term Debt Pre-tax, incremental acquisition costs Business Combination, Acquisition Related Costs Revenue Revenue, Net 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 Decrease of goodwill Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Working Capital Adjustment To Goodwill Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Working Capital Adjustment To Goodwill Expected tax deductible goodwill Business Acquisition, Goodwill, Expected Tax Deductible Amount Acquisition Recap [Abstract] Acquisition Recap [Abstract] Acquisition Recap [Abstract] Purchase Price Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High Less: Long-term debt assumed Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Liabilities, Long-term Debt 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 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 Goodwill Goodwill 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 Public Utilities, Rate Matters, Approved [Abstract] Public Utilities, Rate Matters, Approved [Abstract] Duration Of Base Rate Moratorium Imposed by ASPC Duration Of Base Rate Moratorium Imposed by ASPC Duration Of Base Rate Moratorium Imposed by ASPC Annual Amount of Customer Credit Annual Amount of Customer Credit Annual Amount of Customer Credit Duration Of Annual Customer Credit Duration Of Annual Customer Credit Duration Of Annual Customer Credit Duration of Base Rate Moratorium imposed by CPUC Duration of Base Rate Moratorium imposed by CPUC Duration of Base Rate Moratorium imposed by CPUC Duration of Base Rate Moratorium imposed by NPSC Duration of Base Rate Moratorium imposed by NPSC Duration of Base Rate Moratorium imposed by NPSC Continuation Period of Choice Gas Program Continuation Period of Choice Gas Program Continuation Period of Choice Gas Program Settlement of Gas Supply Contract [Abstract] Settlement of Gas Supply Contract [Abstract] Settlement of Gas Supply Contract [Abstract] Settlement of gas supply contract Early Termination / Settlement Of Gas Supply Contract Early Termination / Settlement Of Gas Supply Contract that required the Company to purchase all of the natural gas produced over the productive life of certain leaseholds. Contract committed to distribution customers, percent Early Termination / Settlement Of Gas Supply Contract, Percentage Of Contract Committed To Distribution Customers Early Termination / Settlement Of Gas Supply Contract, Percentage Of Contract Committed To Distribution Customers Contract not subject to regulatory recovery, percent Early Termination / Settlement Of Gas Supply Contract, Percentage Of Contract Not Subject to Regulatory Recovery Early Termination / Settlement Of Gas Supply Contract, Percentage Of Contract Not Subject to Regulatory Recovery Contract prices Oil and Gas Delivery Commitments and Contracts, Fixed Price Regulatory assets Regulatory Assets Recovery period Regulatory Assets Amortization Period, Unclassified Regulatory Assets settlement, recovery or amortization Period, unclassified as to long-term or short-term. Pro Forma Results Business Acquisition, Pro Forma Information [Abstract] Revenue Business Acquisition, Pro Forma Revenue Net income (loss) available for common stock 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 Notes Payable [Abstract] Schedule of Short-term Debt Schedule of Short-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] Schedule of Credit Facility Covenants Credit Facility and Short Term Debt Covenants [Table Text Block] Schedule of significant debt covenants related to Credit Facility and other short-term debt [Table 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. Deferred energy, fuel and gas cost adjustments - current 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] Asset retirement obligations Asset Retirement Obligation Costs [Member] Loss on reacquired debt Bond Issuance Costs [Member] Utility Bond Issuance Costs recoverable over the life of the original issue, or if refinanced, over the life of the new issue[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 Other regulatory assets Other Regulatory Assets [Member] Other regulatory assets [Member] Regulatory Liability [Axis] Regulatory Liability [Domain] Cost of removal Removal Costs [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 maximum amortization period Regulatory liabilities maximum amortization period Regulatory Liabilities Amortization Period, Unclassified The amortization period for recovery of the individual regulatory liabilities (current and non-current) in a schedule of regulatory liabilities. Regulatory liabilities Regulatory Liabilities Oil and Gas Delivery Commitments and Contracts, Fixed Price 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 Shares Issued Stock Issued During Period, Shares, Dividend Reinvestment Plan Weighted Average Price (usd per share) Dividend Reinvestment Plan, Weighted Average Price Weighted Average Price per Share for shares issued under the Dividend Reinvestment Plan Unissued Shares Available Common Stock, Capital Shares Reserved for Future Issuance Acquisition Business Combination Disclosure [Text Block] Oil and Gas Exploration and Production Industries Disclosures [Abstract] Capitalized Costs of Unproved Properties Excluded from Amortization [Table] Capitalized Costs of Unproved Properties Excluded from Amortization [Table] Oil and Gas Oil and Gas [Member] Oil and Gas [Member] Capitalized Costs of Unproved Properties Excluded from Amortization by Property or Project [Axis] Project [Axis] Unproved Oil and Gas Property or Major Project [Domain] Project [Domain] Current Year Current Year [Member] Current Year [Member] Prior Year Prior Year [Member] Prior Year [Member] More Than One Year, Less Than Two Years Prior More Than One Year, Less Than Two Years Prior [Member] More Than One Year, Less Than Two Years Prior [Member] More Than Two Years Prior More Than Two Years Prior [Member] More Than Two Years Prior [Member] Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items] Capitalized Costs of Unproved Properties Excluded from Amortization [Line Items] Interest Costs, Capitalized During Period Interest Costs Capitalized Leasehold acquisition cost Acquisition Costs, Period Cost Exploration cost Exploration Costs, Period Cost Capitalized interest Capitalized Interest Of Cost Excluded From Amortization The amount of the capitalized interest incurred during the period of unproved properties excluded from amortization. Total Capitalized Costs of Unproved Properties Excluded from Amortization, Period Cost Supplemental Cash Flow Information [Abstract] Supplemental Disclosure of Cash Flow Information Cash Flow, Supplemental Disclosures [Text Block] 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] Commitments and Contingencies Disclosure [Abstract] Loss Contingencies [Table] Loss Contingencies [Table] Electric Utilities Electric Utilities [Member] Electric Utilities Segment [Member] Environmental Remediation Site [Axis] Environmental Remediation Site [Axis] Environmental Remediation Site [Domain] Environmental Remediation Site [Domain] Manufactured Gas Plant Manufactured Gas Plant [Member] Manufactured Gas Plant [Member] Loss Contingencies [Line Items] Loss Contingencies [Line Items] Accrual for Environmental Loss Contingencies Accrual for Environmental Loss Contingencies Insurance Settlements Receivable, Noncurrent Insurance Settlements Receivable, Noncurrent Loss Contingency, Range of Possible Loss Loss Contingency, Estimate of Possible Loss Accrual for Environmental Loss Contingencies, Gross Accrual for Environmental Loss Contingencies, Gross Costs Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities [Table] Costs Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities [Table] Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items] Costs Incurred, Acquisition of Oil and Gas Properties [Abstract] Costs Incurred, Acquisition of Oil and Gas Properties [Abstract] Proved Reserves Costs Incurred, Acquisition of Oil and Gas Properties with Proved Reserves Unproved Reserves Costs Incurred, Acquisition of Unproved Oil and Gas Properties Exploration Costs Costs Incurred, Exploration Costs Development Costs Costs Incurred, Development Costs Asset Retirement Obligation Incurred Costs Incurred, Asset Retirement Obligation Incurred Oil and Gas Property Acquisition, Exploration, and Development Activities Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities Long-term Debt, Unclassified [Abstract] Senior Unsecured Notes Due 2027 Senior Unsecured Notes Due 2027 [Member] Senior Unsecured Notes Due 2027 [Member] Senior Unsecured Notes Due 2046 Senior Unsecured Notes Due 2046 [Member] Senior Unsecured Notes Due 2046 [Member] Senior Unsecured Notes Due 2017 Senior Unsecured Notes Due 2017 [Member] Senior Unsecured Notes Due 2017 [Member] Senior Secured Notes Due 2019 Senior Secured Notes Due 2019 [Member] Senior Secured Notes Due 2019 Corporate Term Loan Due June 2017 Corporate Term Loan Due June 2017 [Member] Corporate Term Loan Due June 2017 [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 April 2017 Corporate Term Loan Due April 2017 [Member] Corporate Term Loan Due April 2017 [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 2026 Senior Unsecured Notes Due 2026 [Member] Senior Unsecured Notes Due 2026 [Member] Senior Unsecured Notes Due 2019 Senior Unsecured Notes Due 2019 [Member] Senior Unsecured Notes Due 2019 Derivative Instrument [Axis] Derivative Instrument [Axis] Derivative Contract [Domain] Derivative Contract [Domain] Interest Rate Swap Interest Rate Swap [Member] Proceeds from senior unsecured notes Proceeds from Issuance of Senior Long-term Debt Interest rate (percent) Debt term Debt Instrument, Term Repayment of debt Extinguishment of Debt, Amount Interest rate swap settlement Gain (Loss) on Sale of Derivatives Notional amount Derivative, Notional Amount Frequency of periodic payment Debt Instrument, Frequency of Periodic Payment Periodic payment Debt Instrument, Periodic Payment Net proceeds from the offering Guarantees [Abstract] Schedule of Guarantor Obligations [Table] Schedule of Guarantor Obligations [Table] Mining Mining [Member] Mining Guarantor Obligations by Nature [Axis] Guarantor Obligations, Nature [Axis] Guarantor Obligations, Nature [Domain] Guarantor Obligations, Nature [Domain] Surety Bond Surety Bond [Member] Guarantor Obligations [Line Items] Guarantor Obligations [Line Items] Guarantor, Maximum Exposure Guarantor Obligations, Maximum Exposure, Undiscounted Guarantor Obligations, Term Guarantor Obligations, Term Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Axis] Defined Benefit Plans and Other Postretirement Benefit Plans [Domain] Defined Benefit Plan and Other Postretirement Benefit Plan [Domain] Supplemental Employee Retirement Plan Supplemental Employee Retirement Plan [Member] Other Postretirement Benefit Plan Other Postretirement Benefit Plan [Member] Pension Plans, Defined Benefit Pension Plan [Member] Healthcare trend rate pre-65 Healthcare trend rate post-65 Black Hills Corporation Pension Plan Black Hills Retirement Plan [Member] Black Hills Retirement Plan [Member] Black Hills Corporation - All Plans Black Hills Service Company Black Hills Service Company [Member] Black Hills Service Company [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] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract] Discount Rate, Benefit Obligation Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate Rate of Increase in Compensation Levels Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Rate of Compensation Increase Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Net Periodic Benefit Cost [Abstract] Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Discount Rate Expected Long-term Return on Assets Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets Rate of Compensation Increase Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase Defined Benefit Plan Assumptions Used In Calculating Net Periodic Benefit Cost Expected Rate of Return On Assets For Next Fiscal Year Defined Benefit Plan Assumptions Used In Calculating Net Periodic Benefit Cost Expected Rate of Return On Assets For Next Fiscal Year An assumption for the next fiscal year as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract] Trend for next year Defined Benefit Plan, Health Care Cost Trend Rate Assumed for Next Fiscal Year Ultimate trend rate Defined Benefit Plan, Ultimate Health Care Cost Trend Rate Year Ultimate Trend Reached Defined Benefit Plan, Year that Rate Reaches Ultimate Trend Rate Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract] Defined Benefit Plan, Effect of One-Percentage Point Change in Assumed Health Care Cost Trend Rates [Abstract] Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation Defined Benefit Plan, Effect of One Percentage Point Increase on Accumulated Postretirement Benefit Obligation Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation Defined Benefit Plan, Effect of One Percentage Point Decrease on Accumulated Postretirement Benefit Obligation Effect of One Percentage Point Increase on Service and Interest Cost Components Defined Benefit Plan, Effect of One Percentage Point Increase on Service and Interest Cost Components Effect of One Percentage Point Decrease on Service and Interest Cost Components Defined Benefit Plan, Effect of One Percentage Point Decrease on Service and Interest Cost Components Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Fair Values Derivatives, Balance Sheet Location, by Derivative Contract Type [Table] Commodity derivatives Commodity Contract [Member] Hedging Designation [Axis] Hedging Designation [Axis] Hedging Designation [Domain] Hedging Designation [Domain] Designated as Hedging Instrument Designated as Hedging Instrument [Member] Derivatives Not Designated as Hedge Instruments Not 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. Derivative Assets, Noncurrent 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. Derivative Liabilities, Noncurrent 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 Asset, Fair Value, Gross Liability Derivative Asset, Fair Value, Gross Liability Derivative Liability, Fair Value, Gross Asset Derivative Liability, 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 Derivative Liability, Fair Value, Net Derivative Liability, Fair Value, Amount Not Offset Against Collateral Schedule of Defined Benefit Plans Disclosures [Table] Schedule of Defined Benefit Plans Disclosures [Table] Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Axis] Supplemental Non-qualified Plans, Defined Benefit Other Pension Plan, Postretirement or Supplemental Plans [Member] Other Postretirement Benefit Plans, Defined Benefit Defined Benefit Plan Disclosure [Line Items] Defined Benefit Plan Disclosure [Line Items] Service cost Defined Benefit Plan, Service Cost Interest cost Defined Benefit Plan, Interest Cost Expected return on assets Defined Benefit Plan, Expected Return on Plan Assets Net amortization of prior service cost Defined Benefit Plan, Amortization of Prior Service Cost (Credit) Recognized net actuarial loss (gain) Defined Benefit Plan, Amortization of Gains (Losses) Settlement Expense Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements and Curtailments Net periodic benefit expense Defined Benefit Plan, Net Periodic Benefit Cost Business Description and Significant Accounting Policies Organization, Consolidation, Basis of Presentation, Business Description and Accounting Policies [Text Block] Document and Entity Information [Abstract] Abstract model section heading for document and entity information Entity Registrant Name Entity Registrant Name Entity Central Index Key Entity Central Index Key Document Type Document Type Document Period End Date Document Period End Date 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 Current Fiscal Year End Date Entity Current Reporting Status Entity Filer Category Entity Filer Category Entity Common Stock, Shares Outstanding Entity Common Stock, Shares Outstanding Entity Well-known Seasoned Issuer Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Voluntary Filers Entity Public Float Entity Public Float Proved Developed and Undeveloped Oil and Gas Reserve Quantities [Table] 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 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] Reserve Quantities [Line Items] Reserve Quantities [Line Items] 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 Proved Developed And Undevleoped 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. 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 Amortization of excess deferred income taxes and investment tax credits Effective Tax Rate Reconciliation, Investment Tax Credits and Excess Deferred The portion of the difference between the effective income tax rate and domestic federal statutory income tax rate that can be explained by investment tax credits and excess deferreds related to plant generated or utilized under enacted tax laws during the period. 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. Other tax differences Effective Income Tax Rate Reconciliation, Other Adjustments, Percent Effective Income Tax Rate, Continuing Operations Effective Income Tax Rate Reconciliation, Percent Statement [Table] Statement [Table] Variable Interest Entity, Primary Beneficiary Variable Interest Entity, Primary Beneficiary [Member] Statement [Line Items] Statement [Line Items] Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Variable Interest Entity, Qualitative or Quantitative Information, Ownership Percentage Notes Payable Short-term Debt [Text Block] Long-term Purchase Commitment [Table] Long-term Purchase Commitment [Table] 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 purchased Power Purchased [Member] Power Purchased [Member] Purchased Gas Cost Obligation Purchased Gas Cost Obligation [Member] Purchased Gas Cost Obligation [Member] Long-term Purchase Commitment [Line Items] Long-term Purchase Commitment [Line Items] Long-term Purchase Commitment Long-term Purchase Commitment [Table Text Block] Unrecorded Unconditional Purchase Obligations Disclosure Unrecorded Unconditional Purchase Obligations Disclosure [Table Text Block] Shares available for grant Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant Unrecognized compensation expense Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized Weighted-average recognition period Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition Stock-based compensation expense Share-based Compensation Credit Facility [Axis] Credit Facility [Axis] Credit Facility [Domain] Credit Facility [Domain] Revolving Credit Facility Revolving Credit Facility [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] Bridge Term Loan Bridge Term Loan - July 2015 [Member] Bridge Term Loan - July 2015 [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] Deferred Financing Costs, Other Deferred Financing Costs, Other [Member] Aggregate amount of deferred financing costs that is associated with Long-Term Debt not separately identified due to immateriality of each individual debt. South Dakota Electric Black Hills Power [Member] Black Hills Power [Member] Wyoming Electric Wyoming Electric [Member] Wyoming Electric Deferred Finance Costs Remaining, Noncurrent Debt Issuance Costs, Noncurrent, Net Amortization expense for deferred financing costs Amortization of Debt Issuance Costs Property, Plant and Equipment [Abstract] Property, Plant and Equipment Property, Plant and Equipment Disclosure [Text Block] Derivative Instruments and Hedging Activities Disclosure [Abstract] Risk Management Activities Derivative Instruments and Hedging Activities Disclosure [Text Block] Unrecorded Unconditional Purchase Obligation [Table] Unrecorded Unconditional Purchase Obligation [Table] Related Party [Axis] Related Party [Axis] Related Party [Domain] Related Party [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 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 Excluded from Computation of Earnings Per Share, by Antidilutive Securities [Axis] Antidilutive Securities [Axis] Antidilutive Securities, Name [Domain] Antidilutive Securities, Name [Domain] Equity Compensation Stock Compensation Plan [Member] Equity Unit Purchase Agreements Equity Unit Purchase Agreements [Member] Antidilutive Securities Excluded from Computation of Earnings Per Share Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items] Outstanding securities not included in the computation of diluted earnings per share (in shares) Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount Segment Reporting Information, Additional Information [Abstract] Schedule of Segment Reporting Information, by Segment [Table] Schedule of Segment Reporting Information, by Segment [Table] Power Generation Power Generation [Member] Power Generation Cash Flow Statement [Axis] Cash Flow Statement [Axis] Cash Flow Statement [Axis] Cash Flow Statement [Domain] Cash Flow Statement [Domain] [Domain] for Cash Flow Statement [Axis] Capital Expenditures Capital Expenditures [Member] Capital Expenditures [Member] Segment Reporting Information [Line Items] Segment Reporting Information [Line Items] Capital Expenditures and Asset Acquisitions Property, Plant and Equipment, Gross, Period Increase (Decrease) Acquisition of net assets, net of long-term debt assumed Property, Plant and Equipment Including New Asset Acquisitions, Gross Period Increase (Decrease) Property, Plant and Equipment Including New Asset Acquisitions, Gross Period Increase (Decrease) Property, Plant and Equipment Including New Asset Acquisitions, Gross Period Increase (Decrease) Prior Period Reclassification Adjustment Prior Period Reclassification Adjustment Oil and Gas Reserves (Unaudited) Oil and Gas Exploration and Production Industries Disclosures [Text Block] Statement of Comprehensive Income [Abstract] Reclassification out of Accumulated Other Comprehensive Income 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] Supplemental Disclosure of Cash Flow Information Schedule of Cash Flow, Supplemental Disclosures [Table Text Block] Intercompany Eliminations Intersegment Eliminations [Member] Operating Segments Operating Segments [Member] Consolidation, Eliminations Consolidation, Eliminations [Member] Segment Reporting Information 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 Costs and Expenses Depreciation, depletion and amortization Depreciation, Depletion and Amortization Impairment of long-lived assets Impairment of Long-Lived Assets Held-for-use Operating income Operating Income (Loss) Interest expense Interest Expense Interest income Investment Income, Interest Other income (expense), net Other Nonoperating Income (Expense) Impairment of equity investments Equity Method Investment, Other than Temporary Impairment Income tax benefit (expense) Income Tax Expense (Benefit) Net income (loss) 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 Accumulated Other Comprehensive Income (Loss) [Table] Accumulated Other Comprehensive Income (Loss) [Table] Commodity Contract Accumulated Net Gain (Loss) fromDesignated 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) [Roll Forward] Increase (Decrease) in Stockholders' Equity [Roll Forward] Accumulated Other Comprehensive Income (Loss), Net of Tax, Period Start Accumulated Other Comprehensive Income (Loss), Net of Tax 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 Accumulated Other Comprehensive Income (Loss), Net of Tax, Period End Supplemental Employee Retirement Plans, Defined Benefit Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax [Abstract] Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax [Abstract] Net (gain) loss Defined Benefit Plan, Accumulated Other Comprehensive Income Net Gains (Losses), after Tax Prior service cost (gain) Defined Benefit Plan, Accumulated Other Comprehensive Income Net Prior Service Cost (Credit), after Tax Total AOCI Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract] Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year [Abstract] Net loss Defined Benefit Plan, Future Amortization of Gain (Loss) Prior service cost (credit) Defined Benefit Plan, Future Amortization of Prior Service Cost (Credit) Total net periodic benefit cost expected to be recognized during calendar year 2017 Defined Benefit Plan, Amount to be Amortized from Accumulated Other Comprehensive Income (Loss) Next Fiscal Year Defined Benefit Plan, Asset Categories [Axis] Defined Benefit Plan, Asset Categories [Axis] Plan Asset Categories [Domain] Plan Asset Categories [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, Target Plan Asset Allocations Current: Current Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Current Federal Tax Expense (Benefit) State Current State and Local Tax Expense (Benefit) Total Current Current Income Tax Expense (Benefit) Deferred: Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract] Federal Deferred Federal Income Tax Expense (Benefit) State Deferred State and Local Income Tax Expense (Benefit) Tax credit amortization Investment Tax Credit Amortization Investment Tax Credit Amortization - The amount deducted from an entity taxes, reflecting a percentage of an amount invested in a certain assets spread over the useful life of those assets: includes deferred investment tax credits. Total Deferred Deferred Income Tax Expense (Benefit) Total Current and Deferred Power Purchase Agreements Power Purchase Agreements [Member] Power Purchase Agreements [Member] Transportation, Storage, Gathering And Coal Agreements Transportation, Storage, Gathering And Coal Agreements [Member] Transportation, Storage, Gathering And Coal Agreements [Member] 2017 Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months 2018 Unrecorded Unconditional Purchase Obligation, Due within Two Years 2019 Unrecorded Unconditional Purchase Obligation, Due within Three Years 2020 Unrecorded Unconditional Purchase Obligation, Due within Four Years 2021 Unrecorded Unconditional Purchase Obligation, Due within Five Years Thereafter Unrecorded Unconditional Purchase Obligation, Due after Five Years Derivative Instruments, Gain (Loss) [Table] Derivative Instruments, Gain (Loss) [Table] Hedging Relationship [Axis] Hedging Relationship [Axis] Hedging Relationship [Domain] Hedging Relationship [Domain] Cash Flow Hedging Cash Flow Hedging [Member] Income Statement Location [Axis] Income Statement Location [Axis] Income Statement Location [Domain] Income Statement Location [Domain] Interest Expense Interest Expense [Member] Sales Revenue, Net Sales Revenue, Net [Member] Fuel, purchased power and cost of natural gas sold Cost of Sales [Member] Not Designated as Hedging Instrument Derivative Instruments, Gain (Loss) [Line Items] Derivative Instruments, Gain (Loss) [Line Items] Summary of Cash Flow Hedge Activity [Abstract] Summary of Cash Flow Hedge Activity [Abstract] Amount of Gain/(Loss) Reclassified from AOCI into Income (Settlements) Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net Amount of Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion) Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net Derivative Instruments Not Designated as Hedging Instruments [Abstract] Derivative Instruments Not Designated as Hedging Instruments [Abstract] Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net Amount of Gain/(Loss) on Derivatives Recognized in Income Derivative, Gain (Loss) on Derivative, Net 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 Wind Farm Busch Ranch Wind Farm [Member] Busch Ranch Wind Project [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 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 Statement of Financial Position [Abstract] Common Stock, Par Value (usd per share) Common Stock, Par or Stated Value Per Share Treasury Stock, Shares Treasury Stock, Shares Common Stock, Shares Authorized Common Stock, Shares Authorized Common Stock, Shares, Issued Common Stock, Shares, Issued Schedule of Company's equity units Schedule of Capital Units [Table Text Block] Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Award Type [Axis] Award Type [Axis] Award Type [Domain] Equity Award [Domain] Restricted Stock Restricted Stock [Member] Performance Shares 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] Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity Schedule of Share-based Compensation, Restricted Stock and Restricted Stock Units Activity [Table Text Block] Disclosure of Share-based Compensation Arrangements by Share-based Payment Award Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block] Schedule of Share-based Compensation, Employee Stock Purchase Plan, Activity Schedule of Dividend Reinvestment Plan [Table Text Block] Schedule describing activity for a dividend reinvestment plan [Table Text Block] Schedule of Variable Interest Entities Schedule of Variable Interest Entities [Table Text Block] 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] Schedule of Oil and Gas In Process Activities Schedule of Oil and Gas In Process Activities [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] Preferred Stock, Shares Authorized Preferred Stock, Shares Authorized Preferred Stock, Shares Outstanding Preferred Stock, Shares Outstanding 2017 Long-term Debt, Current Maturities 2018 Long-term Debt, Maturities, Repayments of Principal in Year Two 2019 Long-term Debt, Maturities, Repayments of Principal in Year Three 2020 Long-term Debt, Maturities, Repayments of Principal in Year Four 2021 Long-term Debt, Maturities, Repayments of Principal in Year Five Thereafter Long-term Debt, Maturities, Repayments of Principal after Year Five Quarterly Financial Information Disclosure [Abstract] Selected Quarterly Financial Information [Table] Selected Quarterly Financial Information [Table] Selected Quarterly Financial Information [Table] Class of Stock [Axis] Class of Stock [Axis] Class of Stock [Domain] Class of Stock [Domain] 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 Income (Loss) from Continuing Operations, Net of Tax, Attributable to Parent Earnings (loss) per share, Basic (usd per share) Earnings Per Share, Basic Earnings (loss) per share, Diluted (usd per share) Earnings Per Share, Diluted Dividends per share paid (usd per share) Common Stock, Dividends, Per Share, Cash Paid Share Price (usd per share) Stock Price Stock Price 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 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. Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Table] Fair Value Measurements, Recurring and Nonrecurring [Table] 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 by 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 [Axis] Fair Value, Hierarchy [Axis] Fair Value, Measurements, Fair Value Hierarchy [Domain] Fair Value Hierarchy [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] 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 Proceeds from Noncontrolling Interests Proceeds from Noncontrolling Interests Assets, Current Assets, Current Property, Plant and Equipment, Gross Property, Plant and Equipment, Gross Liabilities, Current Liabilities, Current 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 Cash and Cash Equivalents, Fair Value Disclosure Restricted cash - carrying amount Restricted Cash and Investments, Current Restricted Cash - fair value 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 Energy [Axis] Energy [Axis] Energy [Domain] Energy [Domain] Crude Oil Crude Oil [Member] Natural Gas, Distribution Natural Gas, Distribution [Member] Natural Gas for distribution [Member] Netting Contract by Category [Axis] Netting Contract, Category [Axis] Netting Contract by Category [Axis] Netting Contract by Category [Domain] Netting Contract, Category [Domain] [Domain] for Netting Contract by Category [Axis] Contract Subject to Master Netting Arrangement Contract Subject to Master Netting Arrangement [Member] Contract Subject to Master Netting Arrangement [Member] Contract Not Subject to Master Netting Arrangement Contract Not Subject to Master Netting Arrangement [Member] Contract Not Subject to Master Netting Arrangement [Member] Basis Swap Basis Swap [Member] Purchase Contract Purchase Contract [Member] Purchase Contract [Member] Derivative Asset [Abstract] Derivative Asset [Abstract] Gross Amounts Offset In Statement Of Financial Position Assets Gross Amounts Offset In Statement Of Financial Position Assets The gross amount of assets offset to determine the net amounts presented in the statement of financial position. Derivative Asset Derivative Liability [Abstract] Derivative Liability [Abstract] Gross Amounts Offset In Statement Of Financial Position Liabilities Gross Amounts Offset In Statement Of Financial Position Liabilities The gross amount of liabilities offset to determine the net amounts presented in the statement of financial position Derivative Liability Other Comprehensive Income Comprehensive Income (Loss) Note [Text Block] Statement of Cash Flows [Abstract] Operating activities: Net Cash Provided by (Used in) Operating Activities [Abstract] Net income (loss) Adjustments to reconcile net income (loss) to net cash provided by operating activities: Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract] Deferred financing cost amortization Amortization of Debt Issuance Costs and Discounts Impairment of long-lived assets and equity method investments Impairment of Oil and Gas Properties Stock compensation Deferred income taxes Employee benefit plans Pension and Other Postretirement Benefit Expense Other adjustments, net Other Noncash Income (Expense) Change 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 current assets Increase (Decrease) in Accounts Receivable and Other Operating Assets Accounts payable and other current liabilities Increase (Decrease) in Other Accounts Payable and Accrued Liabilities Regulatory assets Increase (Decrease) in Other Regulatory Assets Regulatory liabilities Increase (Decrease) in Regulatory Liabilities Contributions to defined benefit pension plans Pension Contributions Interest rate swap settlement Other operating activities, net Increase (Decrease) in Other Operating Assets and Liabilities, Net 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 Proceeds from sale of assets Proceeds from Sale of Other Assets, Investing Activities Other investing activities Payments for (Proceeds from) Other Investing Activities 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 Short-term borrowings - issuances Proceeds from Lines of Credit Short-term borrowings - repayments Repayments of Lines of Credit Long-term debt - repayments Repayments of Long-term Debt Sale of noncontrolling interest Distributions to noncontrolling interests 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 and cash equivalents Cash and Cash Equivalents, Period Increase (Decrease) Cash and cash equivalents: Cash and Cash Equivalents, Period Increase (Decrease) [Abstract] Cash and cash equivalents beginning of year Cash and Cash Equivalents, at Carrying Value, Including Discontinued Operations Cash and cash equivalents end of year 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] Proved Developed and Undeveloped Oil and Gas Reserve Quantities [Table] Oil and Gas Delivery Commitments and Contracts [Axis] Oil and Gas Delivery Commitments and Contracts [Axis] Oil and Gas Delivery Commitments and Contracts, Oil Producing Property [Domain] Oil and Gas Delivery Commitments and Contracts, Oil Producing Property [Domain] Williston Basin Williston Basin [Member] Williston Basin, primarily North Dakota [Member] Powder River Basin Powder River Basin [Member] Powder River Basin [Member] Proved Developed and Undeveloped Reserve Revisions, Cause [Axis] Proved Developed and Undeveloped Reserve Revisions, Cause [Axis] The drivers behind the changes in Proved Developed and Undeveloped Reserves [Axis] Oil and Gas Exploration and Production Disclosure [Domain] Oil and Gas Exploration and Production Disclosure [Domain] Oil and Gas Exploration and Production Disclosure [Domain] Commodity Prices Commodity Prices [Member] Commodity Prices [Member] Reserves Dropped Reserves Dropped [Member] Reserves Dropped [Member] Oil and Gas Well Performance Oil and Gas Well Performance [Member] Oil and Gas Well Performance [Member] Oil and Gas Reserve Revisions, Five Year Reserve Aging Limit Oil and Gas Reserve Revisions, Five Year Reserve Aging Limit [Member] Revisions to Oil and Gas Reserves due to five year reserve aging limitation per SEC guidance [Member] Oil and Gas Reserve Revisions, Reserves Sold Oil and Gas Reserve Revisions, Reserves Sold [Member] Oil and Gas Reserve Revisions, Reserves Sold [Member] Well Development Commencement Period [Axis] Well Development Commencement Period [Axis] The year in which a well was development for which investment is continuing [Axis] Well Development Commencement Period [Domain] Well Development Commencement Period [Domain] [Domain] for The year in which a well was development for which investment is continuing [Axis] Added Reserves Added Reserves [Member] Added Reserves [Member] Wells in Process of Drilling Wells in Process of Drilling Number of States in which Entity Operates Number of States in which Entity Operates Gas and Oil Acreage, Leased Gas and Oil Area, Undeveloped, Net Proved Undeveloped Reserves, Extensions, Discoveries, and Additions Proved Undeveloped Reserves, Extensions, Discoveries, and Additions Additions to proved undeveloped reserves that result from (1) extension of the acreage of previously discovered (old) reservoirs through additional drilling in periods after discovery and (2) discovery of new fields with proved undeveloped reserves or of new reservoirs of proved undeveloped reserves in old fields. Reserves Replaced Reserves Replaced Percentage of reserves replaced through reserve additions. Proved Undeveloped Reserves, Revisions of Previous Estimates Proved Undeveloped Reserves, Revisions of Previous Estimates Revisions represent changes in previous estimates of proved undeveloped reserves, either upward or downward, resulting from new information normally obtained from development drilling and production history or resulting from change in economic factors. Proved Undeveloped Reserve Locations Proved Undeveloped Reserve Locations The quantity of Proved Undeveloped Reserve Locations which the company owned during the year. Proved Undeveloped Reserves Proved Undeveloped Reserves Net quantities of an enterprise's interests in proved undeveloped reserves of either crude oil (including condensate and natural gas liquids), natural gas, synthetic oil and gas, or other renewable natural resource that is intended to be upgraded into synthetic oil and gas as of the beginning and the end of the year. "Net" quantities of reserves include those relating to the enterprise's operating and nonoperating interests in properties. Quantities of reserves relating to royalty interests owned are included in "net" quantities if the necessary information is available to the enterprise. "Net" quantities does not include reserves relating to interests of others in properties owned by the enterprise. The unit of measure for reserve quantities is defined as "barrels" for oil and synthetic oil reserves or "cubic feet" or "cubic meters" for natural gas and synthetic gas reserves. Estimated Future Development Costs of Proved Undeveloped Wells Estimated Future Development Costs of Proved Undeveloped Wells Estimated Future Development Costs of Proved Undeveloped Wells Proved Developed Reserves (Energy) Proved Developed Reserves (Energy) PUD Developed, PUD location PUD Developed, PUD location Number of PUD locations developed during year Capital Expenditure for Proved Undeveloped Reserve Capital Expenditure for Proved Undeveloped Reserve Capital Expenditures during the period to drill and develop Proved Undeveloped Reserve locations. Derivative [Table] Derivative [Table] Derivative [Line Items] Derivative [Line Items] Schedule of Derivative Instruments Schedule of Derivative Instruments [Table 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] Property, Plant and Equipment Property, Plant and Equipment [Member] Property, plant and equipment Prior Period Reclassification Adjustment Subsequent Event Type [Axis] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Subsequent Event Subsequent Event [Member] External Credit Rating by Grouping [Axis] External Credit Rating by Grouping [Axis] External Credit Rating by Grouping [Domain] External Credit Rating by Grouping [Domain] External Credit Rating, Non Investment Grade External Credit Rating, Non Investment Grade [Member] Derivative, by Nature [Axis] Swaps and Options Swaps and Options [Member] A forward-based contract in which two parties agree to swap streams of payments over a specified period, and an option contract in which the underlying asset is a commodity. Options Held Options Held [Member] Swap Swap [Member] Future Future [Member] Commodity Option Commodity Option [Member] Fixed for Float Swaps Purchased Fixed for Float Swaps Purchased [Member] Fixed for Float Swaps Purchased [Member] Purchase Contract Natural Gas Physical Purchases [Member] Natural Gas Physical Purchases [Member] Multiple Interest Rate Swaps [Axis] Multiple Interest Rate Swaps [Axis] Multiple Interest Rate Swaps [Axis] Multiple Interest Rate Swaps [Domain] Multiple Interest Rate Swaps [Domain] [Domain] for Multiple Interest Rate Swaps [Axis] Interest Rate Swap One Interest Rate Swap One [Member] Interest Rate Swap One [Member] Non-investment grade credit exposure Credit Derivative, Maximum Exposure, Undiscounted Notional amount - commodities Derivative, Nonmonetary Notional Amount, Volume Notional amount - commodities Derivative, Nonmonetary Notional Amount, Energy Measure MaximumTerm Hedged in Cash Flow Hedge Maximum Length of Time Hedged in Cash Flow Hedge Loss to be reclassified within twelve months Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months Maximum Term Derivative, Remaining Maturity Effective portion of the loss reclassified from AOCI Derivative Instruments, Loss Reclassified from Accumulated OCI into Income, Effective Portion Amount of Gain/(Loss) Recognized in Income on Derivative (Ineffective Portion) Weighted average fixed interest rate Derivative, Average Fixed Interest Rate Derivative assets, non-current Derivative Asset, Noncurrent Derivative liabilities, current Derivative Liability, Current Derivative liabilities, non-current Derivative Liability, Noncurrent Derivative Expired During the Period Derivative Expired During the Period Derivative Expired During the Period Derivative Expired During Subsequent Period Derivative Expired During Subsequent Period Derivative Expired During Subsequent Period Cash flow hedge loss to be reclassified during next 12 months, Net Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net Subsequent Events [Abstract] Subsequent Events Subsequent Events [Text Block] Schedule of Share-based Compensation Arrangements by Share-based Payment Award [Table] Equity Award [Domain] Employee Stock Option Employee Stock Option [Member] Exercisable at end of period Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Number Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward] Increase (Decrease) in Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves [Roll Forward] Standardized measure - beginning of year Standardized Measure of Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves Sales and transfers of oil and gas produced, net of production costs Sales and Transfers of Oil and Gas Produced, Net of Production Costs Net changes in prices and production costs Net Increase (Decrease) in Sales and Transfer Prices and Production Costs Extensions, discoveries and improved recovery, less related costs Extensions, Discoveries, Additions and Improved Recovery, Less Related Costs Changes in future development costs Changes in Estimated Future Development Costs Development costs incurred during the period Previously Estimated Development Costs Incurred Revisions of previous quantity estimates Revisions of Previous Quantity Estimates Accretion of discount Standardized Measure of Discounted Future Net Cash Flow Relating to Proved Oil and Gas Reserves, Accretion of Discount Net change in income taxes Changes in Future Income Tax Expense Estimates on Future Cash Flows Related to Proved Oil and Gas Reserves Purchases of reserves Increase Due to Purchases of Minerals in Place Sales of reserves Decrease Due to Sales of Minerals in Place Standardized measure - end of year Defined Contribution Plan Name [Axis] Defined Contribution Plan Name [Axis] Defined Contribution Plan Name [Domain] Defined Contribution Plan Name [Domain] Defined Contribution Plan, Company Retirement Defined Contribution Plan, Company Retirement [Member] The Company Retirement portion of the defined contribution plan in which there is no employee contribtuion required to receive a contribution. The contribution is based on a calculation of age and service. [Member] Defined Contribution Plan, 401K Defined Contribution Plan, 401K [Member] The 401K defined contribution plan, which requires a contribution from the employee in order to receive a matching contribution (within limits) from the employer [Member] Defined Benefit Plan, Employer Contributions Defined Contribution Plan, Employer Discretionary Contribution Amount Pension and Other Postretirement Benefit Contributions Pension and Other Postretirement Benefit Contributions Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year Asset Retirement Obligation Disclosure [Abstract] Asset Retirement Obligations [Table] Asset Retirement Obligations [Table] Asset Retirement Obligations [Table] Asset Retirement Obligation Costs [Line Items] Costs provided for the retirement of certain long-live assets that are reasonably expected to be recovered in utility rates charged to customers [Line Items] Asset Retirement Obligation, Roll Forward Analysis [Roll Forward] Beginning Balance Asset Retirement Obligation Liabilities Incurred Asset Retirement Obligation, Liabilities Incurred Liabilities Settled Asset Retirement Obligation, Liabilities Settled Accretion Asset Retirement Obligation, Accretion Expense Liabilities Acquired Asset Retirement Obligation Liabilities Acquired Asset Retirement Obligation Liabilities Acquired Revisions to Prior Estimates Asset Retirement Obligation, Revision of Estimate Ending Balance Retirement of gas pipelines liability Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Regulatory Liability Business Combination Recognized Identifiable Assets Acquired and Liabilities Assumed, Regulatory Liability Estimated change in equipment costs (percent) Asset Retirement Obligation Revision Estimate Percentage Change in Equipment Costs Asset Retirement Obligation Revision Estimate Percentage Change in Equipment Costs Income tax examination [Table] Income Tax Examination [Table] Significant Acquisitions and Disposals by Transaction [Axis] Property, Plant and Equipment, Type [Axis] Significant Acquisitions and Disposals, Transaction [Domain] Property, Plant and Equipment, Type [Domain]