Asset Sale and Exit Activities - Schedule of Retention Benefit Liability Roll Forward (Details) - USD ($) $ in Millions	12 Months Ended		15 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Other current liabilities	$ 156	$ 186	$ 156
Retention Benefits | D.E. Karn Generating Complex
Restructuring Reserve [Roll Forward]
Retention benefit liability at beginning of period	4	0
Costs incurred and charged to maintenance and other operating expenses	13	3	16
Costs incurred and capitalized	2	1
Costs paid or settled	(8)	0
Retention benefit liability at the end of the period	11	4	11
Other current liabilities	$ 3	$ 2	$ 3