Exit Activities - Schedule of Retention Benefit Liability Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended		18 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Other current liabilities	$ 155		$ 155	$ 156
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	4	16
Costs deferred as a regulatory asset	2	0
Retention benefit liability at the end of the period	13	8	13
Other current liabilities	$ 4	$ 4	$ 4