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Commitment, Contingencies And Guarantees: Power Sales Agreements (Details) - MW	12 Months Ended
	Dec. 31, 2021	Jul. 01, 2020	Jun. 26, 2019	Sep. 03, 2014
City Of Gillette
Long-term Purchase Commitment [Line Items]
Megawatts Sold Under Long-Term Contract	23
Purchase Power Contract, MEAN, for up to 20 Megawatts
Long-term Purchase Commitment [Line Items]
Megawatts Sold Under Long-Term Contract	20
2020-2022	15
2022-2023	15
2023-2028	10
Purchase Power Contract, MEAN, for up to 20 Megawatts | Contingent Capacity Amounts on Wygen III
Long-term Purchase Commitment [Line Items]
2020-2022	7
2022-2023	8
2023-2028	5
Purchase Power Contract, MEAN, for up to 20 Megawatts | Contingent Capacity Amounts on Neil Simpson II
Long-term Purchase Commitment [Line Items]
2020-2022	8
2022-2023	7
2023-2028	5
Macquarie Energy, LLC Supply Agreement
Long-term Purchase Commitment [Line Items]
Megawatts Sold Under Long-Term Contract	50
South Dakota Electric | M D U, Montana Dakota Utilities | Contingent Capacity Amounts on Wygen III
Long-term Purchase Commitment [Line Items]
Megawatts Sold Under Long-Term Contract	25
South Dakota Electric | Maximum | M D U, Montana Dakota Utilities
Long-term Purchase Commitment [Line Items]
Megawatts Sold Under Long-Term Contract	50
City of Colorado Springs | Colorado Electric
Long-term Purchase Commitment [Line Items]
Number of Megawatts Capacity Purchased		60
Platte River Power Authority Wind Power Agreement
Long-term Purchase Commitment [Line Items]
Number of Megawatts Capacity Purchased			60
Sharing Arrangement with the City of Gillette, Wyoming
Long-term Purchase Commitment [Line Items]
Long-term Purchase Commitment, Period	20 years
Sharing Arrangement with the City of Gillette, Wyoming | Black Hills Wyoming
Long-term Purchase Commitment [Line Items]
Number of Megawatts Capacity Sold				40

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Number of Megawatts Capacity Purchased

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Number of Megawatts Capacity Sold

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Number of Megawatts Sold Under Long-Term Contract

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- Definition

Total Contract Capacity in Megawatts Due Per Year in Years Five Through Nine of the Contract

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- Definition

Total Contract Capacity in Megawatts Due Per Year in Years Two and Three of the Contract

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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.

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Period covered by the long-term purchase commitment, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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