Summarized Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Regulated and Unregulated Operating Revenue	$ 1,134	$ 1,081	$ 911	$ 1,057	$ 1,060	$ 1,044	$ 910	$ 1,223	$ 4,183	$ 4,237	$ 4,086
Operating income	176	290	161	196	200	284	163	210	823	857	793
Net income	$ 133	$ 218	$ 108	$ 129	$ 133	$ 204	$ 102	$ 139	$ 588	$ 578	$ 552