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	$ 248	$ 251	$ 218	$ 257	$ 232	$ 244	$ 229	$ 276	$ 974	$ 981	$ 1,022
Operating income	31	46	23	41	19	48	22	42	141	131	159
Net income	$ 21	$ 40	$ 12	$ 34	$ 17	$ 29	$ 9	$ 24	$ 107	$ 79	$ 98