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v3.20.2

Notes Payable, Current Maturities and Debt: Debt Covenants (Details) | Sep. 30, 2020
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Line of Credit Facility [Line Items] |
Ratio of Indebtedness to Net Capital | 0.593
Maximum |
Line of Credit Facility [Line Items] |
Consolidated Indebtedness to Capitalization Ratio | 0.65

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- Definition Debt Instrument, Consolidated Indebtedness To Capitalization Ratio Requirement For The Next Fiscal Year + References No definition available. + Details
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Namespace Prefix: | bkh_
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Balance Type: | na
Period Type: | instant

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- 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_LineOfCreditFacilityLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Indebtedness divided by net capital. + References No definition available. + Details
Name: | us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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