Risk Management and Derivatives: Utilities (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2022 USD ($) MMBTU	Dec. 31, 2021 MMBTU
Derivative [Line Items]
Credit Risk Derivative Liabilities, at Fair Value \| $	$ 0.3
Natural Gas, Distribution \| Cash Flow Hedging
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	410,000
Natural Gas, Distribution \| Natural gas futures purchased
Derivative [Line Items]
Derivative, Remaining Maturity	0 months	3 months
Natural Gas, Distribution \| Natural gas futures purchased \| Long
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0	590,000
Natural Gas, Distribution \| Natural gas options purchased, net
Derivative [Line Items]
Derivative, Remaining Maturity	0 months	3 months
Natural Gas, Distribution \| Natural gas options purchased, net \| Long
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0	3,100,000
Natural Gas, Distribution \| Natural gas basis swaps purchased
Derivative [Line Items]
Derivative, Remaining Maturity	0 months	3 months
Natural Gas, Distribution \| Natural gas basis swaps purchased \| Long
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0	870,000
Natural Gas, Distribution \| Natural gas over-the-counter swaps, net
Derivative [Line Items]
Derivative, Remaining Maturity	33 months	34 months
Natural Gas, Distribution \| Natural gas over-the-counter swaps, net \| Long
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	2,750,000	4,570,000
Natural Gas, Distribution \| Natural gas physical contracts, net
Derivative [Line Items]
Derivative, Remaining Maturity	21 months	24 months
Natural Gas, Distribution \| Natural gas physical contracts, net \| Long
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	4,181,531	16,416,677

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5624258-113959

+ Details

Name:	us-gaap_CollateralAlreadyPostedAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

- Definition

Nominal number of units used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 40
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=123477628&loc=d3e90205-114008

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 18
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123467817&loc=SL123496189-112644

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 1I
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123466505&loc=SL123495371-112611

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=125515794&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Period remaining until the derivative contract matures, 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_DerivativeRemainingMaturity1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	srt_EnergyAxis=bkh_NaturalGasDistributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_FutureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PositionAxis=us-gaap_LongMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_BasisSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bkh_FixedforFloatSwapsPurchasedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=bkh_NaturalGasPhysicalPurchasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: