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Regulatory Matters: Gas Supply Contract Termination (Details)	12 Months Ended
	Dec. 31, 2020 $ / Btu
Gas supply contract termination
Regulatory Asset, Amortization Period	5 years
Minimum
Oil and Gas Delivery Commitments and Contracts, Fixed Price	6
Maximum
Oil and Gas Delivery Commitments and Contracts, Fixed Price	8

X

- Definition

The delivery price committed to in the contract.

+ References

No definition available.

+ Details

Name:	us-gaap_OilAndGasDeliveryCommitmentsAndContractsFixedPrice
Namespace Prefix:	us-gaap_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amortization period for the recovery of regulatory asset, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_RegulatoryAssetAmortizationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RegulatoryAssetAxis=bkh_GasSupplyContractTerminationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: