Utility Plant - Jointly Owned Utility Plant (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Plant in Service at Cost	$ 239,341
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	(62,420)
Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Plant in Service at Cost	318,736
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	(98,641)
Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	83
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	(38)
Frederickson 1
Jointly Owned Utility Plant Interests
Plant in Service at Cost	61,790
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	(8,500)
Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	33,831
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	404
Accumulated Depreciation	(7,355)
Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	296,324
Accumulated Depreciation	$ 0
Subsidiaries [Member] \| Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Company’s Ownership Share	50.00%
Plant in Service at Cost	$ 368,242
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	$ (191,323)
Subsidiaries [Member] \| Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Plant in Service at Cost	$ 578,008
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	(357,914)
Subsidiaries [Member] \| Colstrip Units 1 – 4 Common Facilities
Jointly Owned Utility Plant Interests
Plant in Service at Cost	252
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	$ (206)
Subsidiaries [Member] \| Frederickson 1
Jointly Owned Utility Plant Interests
Company’s Ownership Share	49.85%
Plant in Service at Cost	$ 67,858
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	0
Accumulated Depreciation	$ (14,567)
Subsidiaries [Member] \| Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	33.34%
Plant in Service at Cost	$ 47,975
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	404
Accumulated Depreciation	(21,499)
Subsidiaries [Member] \| Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress	130,756
Accumulated Depreciation	0
Other Plant in Service [Member] \| Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Plant in Service at Cost	$ 130,700

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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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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