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Commitments, Contingencies and Guarantees: Power Purchase Agreements (Details) - Solar - TC Colorado Solar, LLC - Colorado Electric	Feb. 19, 2021 MW
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

X

- 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

X

- 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

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

X

- Details

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

X

- Details

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

X

- Details

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