Asset Sale and Exit Activities - Schedule of Retention Benefit Liability Roll Forward (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Restructuring Reserve [Roll Forward]
Other current liabilities	$ 193	$ 193	$ 193	$ 186
Retention Benefits | D.E. Karn Generating Complex
Restructuring Reserve [Roll Forward]
Retention benefit liability at beginning of period		4
Costs incurred and charged to maintenance and other operating expenses	4	11	14
Costs incurred and capitalized		1
Retention benefit liability at the end of the period	16	16	16
Other current liabilities	$ 7	$ 7	$ 7