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	$ 437	$ 615	$ 423	$ 395	$ 428	$ 533	$ 411	$ 454
Operating income	63	163	80	53	55	135	82	75	$ 359	$ 347	$ 311
Net Income (Loss) Attributable to Parent	$ 53	$ 127	$ 72	$ 43	$ 45	$ 105	$ 59	$ 54	$ 295	$ 263	$ 213