Commitments and Contingencies (Details)	3 Months Ended
	Mar. 31, 2021 USD ($)	Dec. 10, 2019 USD ($) MW
Loss Contingencies [Line Items]
Purchase Obligation	$ 777,400,000
Subsidiaries [Member] | Electricity
Loss Contingencies [Line Items]
Change in Load, Percentage	0.10%
Subsidiaries [Member] | Natural Gas
Loss Contingencies [Line Items]
Change in Load, Percentage	(3.70%)
Colstrip Units 1 and 2 [Member]
Loss Contingencies [Line Items]
Ownership interest (percent)	50.00%
Jointly owned utility plant	2
Colstrip Units 3 and 4 [Member]
Loss Contingencies [Line Items]
Ownership interest (percent)	25.00%
Colstrip Unit 4 | Subsidiaries [Member]
Loss Contingencies [Line Items]
Colstrip 4 Sale Amount		$ 1
Colstrip PPA | MW		90