Commitments and Contingencies (Details) $ in Thousands	6 Months Ended
Jun. 30, 2021 USD ($)
Loss Contingencies [Line Items]
Purchase Obligation	$ 802,400
Subsidiaries [Member]
Loss Contingencies [Line Items]
Net Regulatory Assets	$ 15,200
Subsidiaries [Member] \| Electricity
Loss Contingencies [Line Items]
Change in Load, Percentage	0.50%
Subsidiaries [Member] \| Natural Gas
Loss Contingencies [Line Items]
Change in Load, Percentage	(2.00%)
Colstrip Units 1 and 2 [Member]
Loss Contingencies [Line Items]
Ownership interest (percent)	50.00%
Jointly owned utility plant	2
Colstrip Units 3 and 4 [Member]
Loss Contingencies [Line Items]
Ownership interest (percent)	25.00%

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Number of Units

+ References

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- Definition

The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 323
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=114001798&loc=d3e33918-111571

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The net amount of all regulatory assets less all regulatory liabilities as of the end of the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 340
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382

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- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

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