Exit Activities and Discontinued Operations - Schedule of Retention Benefit Liability Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended		24 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Other current liabilities	$ 183	$ 183		$ 183	$ 133
Retention Benefits | D.E. Karn Generating Complex
Restructuring Reserve [Roll Forward]
Retention benefit liability at beginning of period		11	$ 4
Costs incurred and charged to maintenance and other operating expenses		0	11	16
Costs deferred as a regulatory asset	1	5	0
Costs incurred and capitalized		1	1
Retention benefit liability at the end of the period	17	17	16	17
Other current liabilities	$ 5	$ 5	$ 7	$ 5