Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 7,305,530	$ 6,784,679
Less accumulated depreciation and depletion	(1,285,816)	(1,281,493)
Property, plant and equipment of variable interest entities, net	6,019,714	5,503,186
Corporate, Non-Segment
Property, Plant and Equipment [Line Items]
Property, plant and equipment	22,094	29,055
Less accumulated depreciation and depletion	(1,144)	(964)
Property, plant and equipment of variable interest entities, net	20,950	28,091
Property, Plant and Equipment	5,692	5,721
Construction in progress, gross	$ 16,402	$ 23,334
Weighted Average \| Corporate, Non-Segment
Property, Plant and Equipment [Line Items]
Weighted average useful life	10 years	10 years
Minimum \| Corporate, Non-Segment
Property, Plant and Equipment [Line Items]
Weighted average useful life	10 years	3 years
Maximum \| Corporate, Non-Segment
Property, Plant and Equipment [Line Items]
Weighted average useful life	22 years	30 years
Electric Utilities
Property, Plant and Equipment [Line Items]
Total plant in service before construction work in progress	$ 3,159,078	$ 2,956,867
Construction work in progress	89,402	102,268
Property, plant and equipment	3,248,480	3,059,135
Less accumulated depreciation and depletion	(666,669)	(670,861)
Property, plant and equipment of variable interest entities, net	$ 2,581,811	2,388,274
Depreciation, depletion and amortization, remaining amortization period	10 years
Electric Utilities \| Production, Electric
Property, Plant and Equipment [Line Items]
Production, processing and storage	$ 1,417,951	$ 1,348,049
Electric Utilities \| Production, Electric \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	40 years	41 years
Electric Utilities \| Production, Electric \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	32 years
Electric Utilities \| Production, Electric \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	46 years
Electric Utilities \| Electric transmission
Property, Plant and Equipment [Line Items]
Transmission	$ 517,794	$ 483,640
Electric Utilities \| Electric transmission \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	49 years	51 years
Electric Utilities \| Electric transmission \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	44 years
Electric Utilities \| Electric transmission \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	51 years
Electric Utilities \| Electric distribution
Property, Plant and Equipment [Line Items]
Distribution	$ 959,453	$ 861,042
Electric Utilities \| Electric distribution \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	46 years	47 years
Electric Utilities \| Electric distribution \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	46 years
Electric Utilities \| Electric distribution \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	48 years
Electric Utilities \| Plant acquisition adjustment
Property, Plant and Equipment [Line Items]
Plant acquisition adjustment	$ 4,870	$ 4,870
Electric Utilities \| Plant acquisition adjustment \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	32 years	32 years
Electric Utilities \| Plant acquisition adjustment \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	32 years
Electric Utilities \| Plant acquisition adjustment \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	32 years
Electric Utilities \| General
Property, Plant and Equipment [Line Items]
General	$ 259,010	$ 259,266
Electric Utilities \| General \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	28 years	28 years
Electric Utilities \| General \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	26 years
Electric Utilities \| General \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	29 years
Gas Utilities
Property, Plant and Equipment [Line Items]
Total plant in service before construction work in progress	$ 3,245,384	$ 2,918,418
Construction work in progress	67,229	63,080
Property, plant and equipment	3,312,613	2,981,498
Less accumulated depreciation and depletion	(323,679)	(336,721)
Property, plant and equipment of variable interest entities, net	2,988,934	2,644,777
Gas Utilities \| Production, Gas
Property, Plant and Equipment [Line Items]
Production, processing and storage	$ 15,603	$ 13,000
Gas Utilities \| Production, Gas \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	40 years	35 years
Gas Utilities \| Production, Gas \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	24 years
Gas Utilities \| Production, Gas \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	46 years
Gas Utilities \| Gas transmission
Property, Plant and Equipment [Line Items]
Transmission	$ 578,278	$ 516,172
Gas Utilities \| Gas transmission \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	54 years	50 years
Gas Utilities \| Gas transmission \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	22 years
Gas Utilities \| Gas transmission \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	71 years
Gas Utilities \| Gas distribution
Property, Plant and Equipment [Line Items]
Distribution	$ 2,115,082	$ 1,857,233
Gas Utilities \| Gas distribution \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	53 years	43 years
Gas Utilities \| Gas distribution \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	45 years
Gas Utilities \| Gas distribution \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	59 years
Gas Utilities \| Cushion Gas - Depreciable
Property, Plant and Equipment [Line Items]
Production, processing and storage	$ 3,539	$ 3,539
Gas Utilities \| Cushion Gas - Depreciable \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	28 years	28 years
Gas Utilities \| Cushion Gas - Depreciable \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	28 years
Gas Utilities \| Cushion Gas - Depreciable \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	28 years
Gas Utilities \| Cushion Gas - Not Depreciated
Property, Plant and Equipment [Line Items]
Production, processing and storage	$ 39,184	$ 44,443
Gas Utilities \| Gas, Storage
Property, Plant and Equipment [Line Items]
Production, processing and storage	$ 55,481	$ 46,977
Gas Utilities \| Gas, Storage \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	38 years	31 years
Gas Utilities \| Gas, Storage \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	24 years
Gas Utilities \| Gas, Storage \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	52 years
Gas Utilities \| General
Property, Plant and Equipment [Line Items]
General	$ 438,217	$ 437,054
Gas Utilities \| General \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	19 years	20 years
Gas Utilities \| General \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	12 years
Gas Utilities \| General \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	23 years
Power Generation
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 534,803	$ 534,518
Less accumulated depreciation and depletion	(167,787)	(154,362)
Property, plant and equipment of variable interest entities, net	367,016	380,156
Property, Plant and Equipment	529,927	532,397
Construction in progress, gross	$ 4,876	$ 2,121
Power Generation \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	31 years	31 years
Power Generation \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	2 years	2 years
Power Generation \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	40 years	40 years
Mining
Property, Plant and Equipment [Line Items]
Property, plant and equipment	$ 187,540	$ 180,473
Less accumulated depreciation and depletion	(126,537)	(118,585)
Property, plant and equipment of variable interest entities, net	61,003	61,888
Property, Plant and Equipment	186,552	179,198
Construction in progress, gross	$ 988	$ 1,275
Mining \| Weighted Average
Property, Plant and Equipment [Line Items]
Weighted average useful life	14 years	13 years
Mining \| Minimum
Property, Plant and Equipment [Line Items]
Weighted average useful life	2 years	2 years
Mining \| Maximum
Property, Plant and Equipment [Line Items]
Weighted average useful life	59 years	59 years

The depreciation, depletion or amortization period remaining on an asset.

No definition available.

Name:	bkh_DepreciationDepletionAndAmortizationRemainingAmortizationPeriod
Namespace Prefix:	bkh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Property Plant Equipment excluding Construction in Progress

+ References

No definition available.

+ Details

Name:	bkh_PropertyPlantEquipmentExcludingConstructionInProgress
Namespace Prefix:	bkh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Public Utilities, Property, Plant and Equipment Excluding Construction in Progress

+ References

No definition available.

+ Details

Name:	bkh_PublicUtilitiesPropertyPlantandEquipmentExcludingConstructioninProgress
Namespace Prefix:	bkh_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.14)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.

+ References

+ Details

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

- Definition

Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.13)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_PropertyPlantAndEquipmentGross
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_PropertyPlantAndEquipmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(14))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(8))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(13))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Definition

Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment.

+ References

No definition available.

+ Details

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

- Definition

Amount of acquisition adjustments included in property, plant and equipment (PPE) for utilities.

+ References

No definition available.

+ Details

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAmountOfAcquisitionAdjustments1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Period end amount of construction work in progress in public utility.

+ References

No definition available.

+ Details

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

- Definition

Period end amount of property, plant and equipment (PPE) related to utility distribution.

+ References

No definition available.

+ Details

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

- Definition

Period end amount of property, plant and equipment (PPE) related to generation or processing owned by public utility.

+ References

No definition available.

+ Details

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

- Definition

Period end book value of other property, plant and equipment (PPE) owned (but not classified elsewhere) by the public utility.

+ References

No definition available.

+ Details

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

- Definition

Period end amount of property, plant and equipment (PPE) related to utility transmission owned by public utility.

+ References

No definition available.

+ Details

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

- Details

Name:	srt_ConsolidationItemsAxis=us-gaap_CorporateNonSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=bkh_ElectricUtilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ElectricGenerationEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ElectricTransmissionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_ElectricDistributionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=bkh_PlantAcquisitionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_OtherCapitalizedPropertyPlantAndEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=bkh_GasUtilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_GasGatheringAndProcessingEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_GasTransmissionEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_GasDistributionEquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=bkh_CushionGasDepreciableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=bkh_CushionGasNotDepreciatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_NaturalGasStorageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=bkh_PowerGenerationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=bkh_MiningMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: