75000 - Document - Document and Entity Information link:presentationLink link:calculationLink link:definitionLink 75010 - Statement - STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS link:presentationLink link:calculationLink link:definitionLink 75020 - Statement - STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS link:presentationLink link:calculationLink link:definitionLink 75030 - Disclosure - Description of the Plan link:presentationLink link:calculationLink link:definitionLink 75040 - Disclosure - Summary of Significant Accounting Policies link:presentationLink link:calculationLink link:definitionLink 75050 - Disclosure - Fair Value Measurements link:presentationLink link:calculationLink link:definitionLink 75060 - Disclosure - Plan Termination link:presentationLink link:calculationLink link:definitionLink 75070 - Disclosure - Federal Income Tax Status link:presentationLink link:calculationLink link:definitionLink 75080 - Disclosure - Related Party Transactions and Exempt Party-in-Interest Transactions link:presentationLink link:calculationLink link:definitionLink 75090 - Disclosure - Pending Merger Between BHC and Northwestern link:presentationLink link:calculationLink link:definitionLink 75100 - Disclosure - Subsequent Events link:presentationLink link:calculationLink link:definitionLink 75110 - Disclosure - Schedule of Assets (Held at End of Year) link:presentationLink link:calculationLink link:definitionLink 75120 - Disclosure - Summary of Significant Accounting Policies (Policies) link:presentationLink link:calculationLink link:definitionLink 75130 - Disclosure - Fair Value Measurements (Tables) link:presentationLink link:calculationLink link:definitionLink 75140 - Disclosure - Schedule of Assets (Held at End of Year) (Tables) link:presentationLink link:calculationLink link:definitionLink 75150 - Disclosure - Description of the Plan - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75160 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75170 - Disclosure - Fair Value Measurements - Summary of Investments Measured at Fair Value on Recurring Basis (Details) link:presentationLink link:calculationLink link:definitionLink 75180 - Disclosure - Plan Termination - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75190 - Disclosure - Federal Income Tax Status - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75200 - Disclosure - Related Party Transactions and Exempt Party-in-Interest Transactions - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75210 - Disclosure - Pending Merger Between BHC and Northwestern - Additional Information (Details) link:presentationLink link:calculationLink link:definitionLink 75220 - Disclosure - Schedule of Assets (Held at End of Year) - Schedule of Assets (Held at End of Year) (Details) link:presentationLink link:calculationLink link:definitionLink 75220 - Disclosure - Schedule of Assets (Held at End of Year) - Schedule of Assets (Held at End of Year) (Details) [Default] link:presentationLink link:calculationLink link:definitionLink EBP, Change in Net Asset Available for Benefit, Increase [Abstract] Additions to net assets Employee benefit plan employer participant pre tax, roth and after tax deferral contributions matched percentage Employee Benefit Plan Employer Participant Pre tax, Roth and After tax Deferral Contributions Matched Percentage Employee benefit plan employer participant pre tax, roth and after tax deferral contributions matched percentage. Benefits paid to participants EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant Benefits paid to participants EBP, Receivable [Abstract] Receivables: Fair Value, Inputs, Level 2 [Member] Level 2 EBP, Related Party and Party-in-Interest Transactions [Line Items] Document Fiscal Period Focus EBP, Employer Contribution, Receivable Employer contribution EBP, Forfeited Nonvested Account, Decrease from Employer Contribution Reduction of employer contributions Employee Benefit Plan Employer Contribution Participant Compensation Matched Amount Employee benefit plan employer contribution participant compensation matched amount. Employee benefit plan employer contribution participant compensation matched amount EBP, Tax Status [Abstract] Employee Benefit Plan Years of Service Rate Vesting of Employee Contribution Employee benefit plan years of service rate vesting of employee contribution. Employee benefit plan years of service rate vesting of employee contribution Forfeited nonvested accounts EBP, Forfeited Nonvested Account EBP, Description of Plan [Text Block] Description of the Plan EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant Interest received on notes receivable from participants Investment, Identifier [Axis] EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense Administrative expenses Administrative expenses paid by Plan participants and by Plan forfeitures EBP, Description of Plan [Line Items] EBP, Investment, Issuer Name [Extensible Enumeration] EBP, Investment, Fair Value and NAV [Abstract] EBP, Accounting Policy [Line Items] EBP, Schedule of Asset Held for Investment [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] Employee Benefit Plan, Conversion of Shares into Rights to Receive Shares Employee benefit plan, conversion of shares into rights to receive shares. Right to receive validly issued, fully paid and non-assessable shares of common stock Note receivable from participant, account, maximum borrowing, amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Uncertain tax positions taken Employee Benefit Plan Uncertain Tax Positions Taken Employee benefit plan, uncertain tax positions taken. Common Collective Trust [Member] Common Collective Trust Funds Amendment Flag Employer contributions EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash, Total EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] Deductions from net assets: EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Note receivable from participant, account, minimum borrowing, amount Employee Benefit Plan, Pending Merger Between BHC and Northwestern [Table] Employee benefit plan, pending merger between BHC and northwestern table. Common Stock [Member] Common Stock NorthWestern Energy Group, Inc. ("NorthWestern") and River Merger Sub Inc. ("Merger Sub") [Member] NorthWestern Energy Group, Inc. ("NorthWestern") and River Merger Sub Inc. ("Merger Sub") member. NorthWestern Energy Group, Inc. ("NorthWestern") and River Merger Sub Inc. ("Merger Sub") Employee Benefit Plan Participant Maximum Savings Rate Percentage EmpEmployee benefit plan participant maximum savings rate percentage. Employee benefit plan participant maximum savings rate percentage EBP, Schedule of Asset Held for Investment [Line Items] Fair Value Hierarchy and NAV [Axis] EBP, Investment, Current Value, Type [Extensible Enumeration] Investment Type [Axis] Document Type Entity Central Index Key Employee Benefit Plan Employer Retirement Contribution Participant Amount Employee benefit plan employer retirement contribution participant amount. Employee benefit plan employer retirement contribution participant amount Self-Directed Brokerage Accounts - PCRA EBP, Self-Directed Brokerage Account [Member] Investment Valuation and Income Recognition EBP, Investment [Policy Text Block] EBP, Contribution, Participating Employer [Axis] Employee Benefit Plan, Common Stock, Par Value Employee benefit plan, common stock, par value. Common stock, par value EBP, Subsequent Event [Table] EBP, Accounting Policy [Table] EBP, Subsequent Event [Abstract] EBP, Basis of Accounting [Policy Text Block] Basis of Accounting Employee benefit plan participant contribution pretax, aftertax amount Employee Benefit Plan Participant Contribution Pretax, Aftertax Amount Employee benefit plan participant contribution pretax, aftertax amount. EBP, Description of Plan [Table] Validly Issued, Fully Paid and Non-Assessable Shares [Member] validly issued, fully paid and non-assessable shares, member. Validly Issued, Fully Paid and Non-Assessable Shares EBP, Employer, Common Stock [Member] Black Hills Corporation Common Stock EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment, Total Interest and dividends EBP, Investment, Fair Value [Abstract] Investments - at fair value: EBP, Noninterest-Bearing Cash Cash EBP, Investment, Number of Shares Shares of common stock held by the plan Minimum [Member] Minimum EBP, Related Party and Party-in-Interest Transactions [Text Block] Related Party Transactions and Exempt Party-in-Interest Transactions Total additions to net assets EBP, Change in Net Asset Available for Benefit, Increase Fair Value, Inputs, Level 3 [Member] Level 3 EBP, Change in Net Asset Available for Benefit, Increase from Contribution Total contributions EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Participation Status [Extensible Enumeration] Employee Benefit Plan, Pending Merger Between BHC and Northwestern [Line Items] Employee benefit plan, pending merger between BHC and northwestern, line items. Employee Benefit Plan Participant Contribution Pretax, Aftertax Eligible Annual Pay Percentage Employee benefit plan participant contribution pretax, aftertax eligible annual pay percentage. Employee benefit plan participant contribution pretax, aftertax eligible annual pay percentage EBP, Investment Sold, Excluding Plan Interest in Master Trust, Selling Price Market value EBP, Expense [Policy Text Block] Administrative Expenses Employee Benefit Plan Change In Net Asset Available For Benefit Increase (Decrease) For Other Income (Loss)[Abstract] Employee benefit plan change in net asset available for benefit increase (decrease) for other income (loss). Other income: Document Information [Line Items] Fair Value Measured at Net Asset Value Per Share [Member] Net Asset Value EBP, Statement of Net Asset Available for Benefit [Abstract] Employee Benefit Plan Participant Account Balance Percentage Employee benefit plan participant account balance percentage. Employee benefit plan participant account balance percentage EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Participant contributions Fair Value, Inputs, Level 1, Level 2, and Level 3 [Member] Fair Value, Inputs, Level 1, Level 2, and Level 3 EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] Contributions: EBP, Tax Determination Letter, Date Tax determination letter date EBP, Contribution, Participating Employer [Domain] EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value, Ending Balance EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value, Beginning Balance Total Investments Measured at Fair Value Participant-directed investments Investments held EBP, Use of Estimate [Policy Text Block] Use of Estimates Mutual Fund [Member] Mutual Funds Entity [Domain] EBP, Investment Acquired, Excluding Plan Interest in Master Trust, Cost Cost basis Employee Benefit Plan, Pending Merger Between BHC and Northwestern [Abstract] Employee benefit plan, pending merger between BHC and northwestern, abstract. EBP, Summary of Accounting Policy [Text Block] Summary of Significant Accounting Policies Employee Benefit Plan, Pending Merger Between BHC and Northwestern Employee Benefit Plan, Pending Merger Between BHC and Northwestern [Text Block] Employee benefit plan, pending merger between BHC and northwestern, text block. EBP, Investment, Fair Value and NAV [Text Block] Fair Value Measurements Charles Schwab [Member] Charles Schwab member. Charles Schwab EBP, Description of Plan and Accounting Change [Abstract] EBP, Asset Held for Investment, Participant Loan, after Deemed Distribution Participant loans, Current Value EBP, Statement of Change in Net Asset Available for Benefit [Line Items] Statistical Measurement [Domain] Excess Contributions Payable EBP, Excess Contribution Payable [Policy Text Block] EBP, Note Receivable from Participant Notes receivable from participants Document Period End Date Fair Value Hierarchy and NAV [Domain] EBP, Risk and Uncertainty [Policy Text Block] Risks and Uncertainties EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Note receivable from participant, account, maximum borrowing, to total account, percentage EBP, Tax Determination Letter, Obtained [true false] Tax opinion letter, obtained EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Plan Termination [Text Block] Plan Termination EBP, Subsequent Event [Text Block] Subsequent Events EBP, Participant Contribution, Pretax, Maximum Annual Compensation, Amount Employee benefit plan annual maximum pre-tax contribution amount Cover [Abstract] Employee Benefit Plan Note Receivable from Participant Interest Rates Employee benefit plan note receivable from participant interest rates. Note receivable from participant interest rates Investments [Domain] EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] Investment income: EBP, Net Asset Available for Benefit Beginning of year End of year Net assets available for benefits EBP 003 EBP 003 [Member] EBP 003. EBP, Investment, Fair Value and NAV [Table Text Block] Summary of Investments Measured at Fair Value on Recurring Basis EBP, Schedule of Asset Held for Investment [Abstract] EBP, Excess Contribution Payable Excess contributions payable Employee benefit plan deferral rate percentage Employee Benefit Plan Deferral Rate Percentage Employee benefit plan deferral rate percentage. Current Value EBP, Asset Held for Investment, Investment, Excluding Plan Interest in Master Trust, Current Value Employee Benefit Plans Vested Percentage Participants Employee benefit plans vested percentage participants. Employee benefit plans vested percentage participants Notes Receivable from Participants EBP, Note Receivable from Participant [Policy Text Block] Employee Benefit Plan, Payment of Fees for Trust and Record Keeping Services Employee benefit plan, payment of fees for trust and record keeping services. Payment of fees for trust and record keeping services EBP, Schedule of Asset Held for Investment [Table Text Block] Schedule of Assets (Held at End of Year) EBP, Statement of Net Asset Available for Benefit [Table] Document Fiscal Year Focus Employee benefit plan matching contributions percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Contribution [Policy Text Block] Contributions EBP, Participant Contribution, Pretax, Maximum Annual Compensation to Total Compensation, Percentage Employee benefit plan participant contribution percentage Money Market Funds [Member] Money Market Funds Plan administrative expenses incurred by Company EBP, Administrative Expenses Employee benefit plan administrative expenses. EBP, Plan Number EBP, Investment, Fair Value and NAV [Line Items] Document Information [Table] Legal Entity [Axis] EBP, Plan Name EBP, Asset [Abstract] Assets: EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment Total investment income Employee Benefit Plan Participant Catch up Contributions Amount EmpEmployee benefit plan participant catch up contributions amount. Employee benefit plan participant catch up contributions amount EBP, Investment, Fair Value and NAV [Table] EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Participant rollover contributions Schwab U.S. Treasury Money Fund [Member] Schwab U.S. Treasury Money Fund. Schwab U.S. Treasury Money Fund EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] EBP, Asset Held for Investment EBP, Asset Held for Investment, Total Total Current Value EBP, Subsequent Event [Line Items] EBP, Tax Status [Text Block] Federal Income Tax Status Maximum [Member] Maximum Statistical Measurement [Axis] EBP, Statement of Change in Net Asset Available for Benefit [Abstract] Employee Benefit Plan Change In Net Asset Available For Benefit Increase From Net Appreciation In Fair Value Of Investments Employee benefit plan change in net asset available for benefit increase from net appreciation in fair value of investments. Net appreciation in fair value of investments Dividend income EBP, Change in Net Asset Available for Benefit, Increase from Dividend Income on Investment Entity Registrant Name EBP, Receivable Total receivables EBP, Asset Held for Investment, Participant Loan, Interest Rate Participant loans, interest rates Schwab Personal Choice Retirement Account [Member] Schwab Personal Choice Retirement Account. EBP, Related Party and Party-in-Interest [Domain] EBP, Related Party and Party-in-Interest [Axis] EBP, Roth 401(k) Option [Policy Text Block] Employee benefit plan, Roth 401(k) Option. Roth 401(k) Option EBP, Change in Net Asset Available for Benefit, Decrease Total deductions from net assets Total deductions from net assets Notes Receivable from Participants [Member] Notes receivable from participants, member. Notes Receivable from Participants Employee Benefit Plan Statement Of Change In Net Asset Available For Benefits [Abstract] Employee benefit plan statement of change in net asset available for benefits. Net assets available for benefits: Fair Value, Inputs, Level 1 [Member] Level 1 Employee Benefit Plan Schedule Of Asset Held For Investments Text Block Employee benefit plan schedule of assets held for investments. Schedule of Assets (Held at End of Year) Employee Benefit Plan Plan Termination Participants Percentage Vested In Accounts Employee benefit plan plan termination participants percentage vested in accounts. Percentage vested by participants, if Plan is terminated EBP, Tax Status [Line Items] Entity Tax Identification Number EBP, Asset Held for Investment, Participant Loan, Maturity Date Participant loans, maturity date Employee Benefit Plan Participant Savings Rate Percentage Increase Annually Employee benefit plan participant savings rate percentage increase annually. Employee benefit plan participant savings rate percentage increase annually EBP, Change in Net Asset Available for Benefit, Increase (Decrease) Increase in net assets available for benefits EBP, Tax Status [Table] EBP, Asset Held for Investment, Party-in-Interest [true false] Vanguard Target Retirement 2055 Fund TR I [Member] Vanguard Target Retirement 2025 Fund TR I [Member] Vanguard Target Retirement 2065 Fund TR I [Member] Vanguard Inflation-Protected Securities Fund [Member] Vanguard Target Retirement 2020 Fund TR I [Member] Vanguard Target Retirement 2045 Fund TR I. Vanguard Target Retirement 2040 Fund TR I [Member] Vanguard Target Retirement Income Fund TR I [Member] Invesco Stable Value CL B1. Vanguard target retirement 2055 fund TR I. Vanguard Target Retirement 2070 Fund TR I [Member] Vanguard total international stock index. Vanguard inflation-protected securities fund. Vanguard Target Retirement 2025 Fund TR I. Vanguard Total International Stock Index [Member] Vanguard Target Retirement 2050 Fund TR I [Member] Vanguard Target Retirement 2020 Fund TR I. Invesco Stable Value CL B1 [Member] Black Hills Corporation1 [Member] Vanguard Target Retirement 2035 Fund TR I [Member] Vanguard target retirement 2070 fund TR I. Vanguard Total Bond Market Index Fund [Member] Vanguard total bond market index fund. Vanguard Target Retirement 2035 Fund TR I. Vanguard Target Retirement 2060 Fund TR I [Member] Vanguard target retirement 2060 fund TR I. Vanguard Institutional Index Plus Fund [Member] Vanguard Target Retirement 2045 Fund TR I [Member] Vanguard target retirement income fund TR I. Vanguard Target Retirement 2030 Fund TR I [Member] Black Hills Corporation. Vanguard Target Retirement 2040 Fund TR I. Vanguard target retirement 2065 fund TR I. Vanguard REIT Index Fund [Member] Vanguard Target Retirement 2030 Fund TR I. Vanguard Extended Market Index Fund [Member] Vanguard REIT index fund. Vanguard institutional index plus fund. Vanguard Target Retirement 2050 Fund TR I. Vanguard extended market index fund.