Jointly Owned Electric Facilities - Schedule of Utility Plant in Service for Colstrip And Accumulated Depreciation (Details) - Colstrip Generating Project [Member] - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Jointly Owned Utility Plant Interests [Line Items]
Utility plant in service	$ 391,922	$ 387,860
Accumulated depreciation	$ (284,282)	$ (268,637)

The period end amount owned by the reporting utility of a jointly owned utility plant in service.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=115995737&loc=d3e659983-123033

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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.

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- Definition

The period end amount of accumulated depreciation in a jointly owned electricity generation plant.

+ References

+ Details

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- Details

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