Litigation (Details)	12 Months Ended
	Dec. 31, 2020 unit	Dec. 10, 2019 USD ($) MW
Jointly Owned Utility Plant Interests
Clean Air Rule, Emissions Cap, Percentage	(5.00%)
Clean Air Rule, Emissions Cap, Years	3 years
Subsidiaries [Member] | Colstrip Unit 4 [Member]
Jointly Owned Utility Plant Interests
Company’s Ownership Share	25.00%
Colstrip 4 Sale Amount | $		$ 1
Colstrip PPA | MW		90
Subsidiaries [Member] | Colstrip Units 1 & 2
Jointly Owned Utility Plant Interests
Company’s Ownership Share	50.00%
Number of Units | unit	2