Variable Interest Entities - PAC (Details) - PAC - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Variable interest entity, ownership percentage	66.67%
Net assets	$ 45	$ 74
Jim Bridger Nos. 1-4
Schedule of Equity Method Investments [Line Items]
Ownership share	67.00%
Jim Bridger Nos. 1-4 | Not Primary Beneficiary
Schedule of Equity Method Investments [Line Items]
Ownership share	66.67%
Jointly owned utility plant, joint owner share	33.33%
Share of coal production purchased by Company	66.67%
Share of coal production purchased by joint venture partner	33.33%