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Employee Benefit Plans: Defined Contribution Plans (Details) | 12 Months Ended
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Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items] |
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent | 50.00%
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage | 20.00%
Defined Contribution Plan, Employee Vesting Period | 5 years
Maximum |
Defined Benefit Plan Disclosure [Line Items] |
Defined Contribution Plan, Employers Matching Contribution, Annual Vesting Percentage | 100.00%

X
- Definition Period over which an employee vests in a defined contribution plan. + References No definition available. + Details
Name: | bkh_DefinedContributionPlanEmployeeVestingPeriod
Namespace Prefix: | bkh_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |