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Commitments and Contingencies: Future Purchase Agreement - Related Party (Details) - MW | Sep. 11, 2020 | Jul. 01, 2020 | Jun. 26, 2019
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Platte River Power Authority Wind Power Agreement |  |  |
Unrecorded Unconditional Purchase Obligation [Line Items] |  |  |
Number of Megawatts Capacity Purchased |  |  | 60
Colorado Electric | City of Colorado Springs |  |  |
Unrecorded Unconditional Purchase Obligation [Line Items] |  |  |
Number of Megawatts Capacity Purchased |  | 60 |
South Dakota Electric | Fall River Solar |  |  |
Unrecorded Unconditional Purchase Obligation [Line Items] |  |  |
Number of Megawatts Capacity Purchased | 80 |  |
Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility | 20 years |  |

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- Definition Number of Megawatts Capacity Purchased + References No definition available. + Details
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Namespace Prefix: | bkh_
Data Type: | num:powerItemType
Balance Type: | na
Period Type: | instant

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- Definition Purchase Power Agreement Set to Expire after a Certain Number of Years Following Completion of the Facility + References No definition available. + Details
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Namespace Prefix: | bkh_
Data Type: | xbrli:durationItemType
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- 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
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Namespace Prefix: | us-gaap_
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |