Risk Management and Derivatives: Utilities (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020 USD ($) MMBTU mW	Dec. 31, 2019 MMBTU mW
Natural Gas, Distribution \| Cash Flow Hedging
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	800,000
Natural Gas, Distribution \| Natural gas futures purchased
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	920,000	1,450,000
Derivative, Remaining Maturity	9 months	12 months
Natural Gas, Distribution \| Natural gas options purchased, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	0	3,240,000
Derivative, Remaining Maturity	0 months	3 months
Natural Gas, Distribution \| Natural gas basis swaps purchased
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	790,000	1,290,000
Derivative, Remaining Maturity	9 months	12 months
Natural Gas, Distribution \| Natural gas over-the-counter swaps, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	4,620,000	4,600,000
Derivative, Remaining Maturity	26 months	24 months
Credit Risk Derivative Liabilities, at Fair Value \| $	$ 0.5
Natural Gas, Distribution \| Natural gas physical contracts, net
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	1,104,725	13,548,235
Derivative, Remaining Maturity	12 months	12 months
Electricity \| Electric wholesale contracts
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount \| mW	195,825	0
Derivative, Remaining Maturity	9 months	0 months

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=109980867&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/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 2: 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=109980867&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_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:

- Details

Name:	srt_EnergyAxis=us-gaap_ElectricityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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