Supplemental Quarterly Results (Schedule of Quarterly Financial Inform) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Supplemental Quarterly Results [Abstract]
Operating Revenues	$ 1,175	$ 951	$ 816	$ 920	$ 809	$ 737	$ 811	$ 846	$ 3,862	$ 3,203	$ 2,436
Operating income	352	66	124	255	167	110	188	266	797	731	(2,190)
Net income (loss) attributable to common stock	$ 307	$ (29)	$ 51	$ 205	$ 267	$ 43	$ 224	$ 281	$ 535	$ 815	$ (2,751)
Earnings (loss) per share: -Basic	$ 0.54	$ (0.05)	$ 0.09	$ 0.36	$ 0.53	$ 0.09	$ 0.45	$ 0.57	$ 0.93	$ 1.64	$ (6.32)
Earnings (loss) per share - Diluted	$ 0.54	$ (0.05)	$ 0.09	$ 0.36	$ 0.53	$ 0.09	$ 0.45	$ 0.57	$ 0.93	$ 1.63	$ (6.32)