Utility Plant - Jointly Owned Utility Plant (Details) $ in Thousands	Dec. 31, 2021 USD ($)
Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Plant in Service at Cost	$ 339,073
Construction Work in Progress	0
Accumulated Depreciation	(153,950)
Frederickson 1
Jointly Owned Utility Plant Interests
Plant in Service at Cost	63,210
Construction Work in Progress	0
Accumulated Depreciation	(18,215)
Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	42,736
Construction Work in Progress	471
Accumulated Depreciation	(10,867)
Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Construction Work in Progress	484,299
Accumulated Depreciation	$ 0
Subsidiaries [Member] \| Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Plant in Service at Cost	$ 597,009
Construction Work in Progress	0
Accumulated Depreciation	$ (411,887)
Subsidiaries [Member] \| Frederickson 1
Jointly Owned Utility Plant Interests
Company’s Ownership Share	49.85%
Plant in Service at Cost	$ 69,278
Construction Work in Progress	0
Accumulated Depreciation	$ (24,283)
Subsidiaries [Member] \| Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	33.34%
Plant in Service at Cost	$ 56,820
Construction Work in Progress	471
Accumulated Depreciation	(24,952)
Subsidiaries [Member] \| Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Construction Work in Progress	239,566
Accumulated Depreciation	$ 0

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=122136288&loc=d3e659983-123033

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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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- Definition

The period end amount of construction work in progress at a jointly owned electricity generation plant.

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- Definition

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

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- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

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