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Financing: At-the-Market Equity Offering Program (Additional Information) (Details) - USD ($) $ in Millions | Jun. 16, 2023 | Aug. 04, 2020
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Line of Credit Facility [Line Items] |  |
At the market equity offering program authorized aggregate value | $ 400 | $ 400

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- Definition At The Market Equity Offering Program authorized an aggregate value that the Company may, through its sales agents, offer and sell from time to time shares of its common stock. + References No definition available. + Details
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Namespace Prefix: | bkh_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 235 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(f)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480678/235-10-S99-1 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration