Utility Plant - Jointly Owned Utility Plant (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Plant in Service at Cost	$ 323,100
Construction Work in Progress	0
Accumulated Depreciation	(138,827)
Frederickson 1
Jointly Owned Utility Plant Interests
Plant in Service at Cost	61,820
Construction Work in Progress	0
Accumulated Depreciation	(10,995)
Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	36,837
Construction Work in Progress	119
Accumulated Depreciation	(8,452)
Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Construction Work in Progress	362,684
Accumulated Depreciation	0
Subsidiaries [Member] | Colstrip Units 1 and 2 [Member]
Jointly Owned Utility Plant Interests
Regulatory Assets, Unrecovered plant balance	$ 126,500
Subsidiaries [Member] | Colstrip Units 3 & 4
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Plant in Service at Cost	$ 582,372
Construction Work in Progress	0
Accumulated Depreciation	$ (398,099)
Subsidiaries [Member] | Frederickson 1
Jointly Owned Utility Plant Interests
Company’s Ownership Share	49.85%
Plant in Service at Cost	$ 67,888
Construction Work in Progress	0
Accumulated Depreciation	$ (17,063)
Subsidiaries [Member] | Jackson Prairie [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	33.34%
Plant in Service at Cost	$ 50,963
Construction Work in Progress	119
Accumulated Depreciation	(22,578)
Subsidiaries [Member] | Tacoma LNG [Member]
Jointly Owned Utility Plant Interests
Plant in Service at Cost	0
Construction Work in Progress	162,820
Accumulated Depreciation	$ 0