Supplemental Quarterly Results (Schedule of Quarterly Financial Inform) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Operating Revenues	$ 745	$ 636	$ 667	$ 990	$ 1,175	$ 951	$ 816	$ 920	$ 3,038	$ 3,862	$ 3,203
Operating income	64	(29)	22	213	352	66	124	255	270	797	731
Net income (loss) attributable to common stock	$ 110	$ 49	$ 138	$ 594	$ 307	$ (29)	$ 51	$ 205	$ 891	$ 535	$ 815
Earnings (loss) per share: -Basic	$ 0.20	$ 0.09	$ 0.26	$ 1.10	$ 0.54		$ 0.09	$ 0.36	$ 1.65	$ 0.93	$ 1.64
Earnings (loss) per share - Diluted	$ 0.20	$ 0.09	$ 0.26	$ 1.10	$ 0.54	$ (0.05)	$ 0.09	$ 0.36	$ 1.65	$ 0.93	$ 1.63