Exit Activities and Discontinued Operations - Schedule of Retention Benefit Liability Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended		18 Months Ended
	Jun. 30, 2021	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020
Restructuring Reserve [Roll Forward]
Other current liabilities	$ 174	$ 174			$ 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	7	$ 16
Costs deferred as a regulatory asset	2	4	0
Costs incurred and capitalized		0	1
Retention benefit liability at the end of the period	15	15	12
Other current liabilities	$ 5	$ 5	$ 5