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Commitments, Contingencies and Guarantees: Power Purchase Agreements (Details) - Solar - TC Colorado Solar, LLC - Colorado Electric | Feb. 19, 2021 MW
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Unrecorded Unconditional Purchase Obligation [Line Items] |
Number of Megawatts Capacity Purchased | 200
Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility | 15 years

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- Definition Number of Megawatts Capacity Purchased + References No definition available. + Details
Name: | bkh_NumberOfMegawattsCapacityPurchased
Namespace Prefix: | bkh_
Data Type: | dtr-types:powerItemType
Balance Type: | na
Period Type: | instant

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- Definition Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility + References No definition available. + Details
Name: | bkh_PurchasePowerAgreementSetToExpireAfterACertainNumberOfYearsFollowingCompletionOfTheFacility
Namespace Prefix: | bkh_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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

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- Details
Name: | bkh_FuelTypeAxis=bkh_SolarMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_LongTermContractForPurchaseOfElectricPowerAxis=bkh_TCColoradoSolarLLCMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | dei_LegalEntityAxis=bkh_ColoradoElectricMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |