Quarterly Financial Information (Schedule of Quarterly Financial Information) (Details) - USD ($) $ 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 Information Disclosure [Abstract]
Operating revenue	$ 771	$ 795	$ 698	$ (335)	$ 689	$ 739	$ 632	$ 702
Operating income (loss)	(76)	261	17	(1,143)	(1,271)	212	107	121	$ (941)	$ (831)	$ (83)
Total comprehensive income (loss)	(191)	143	(70)	(1,103)	(852)	104	31	128	$ (1,221)	$ (588)	$ (172)
Comprehensive income (loss) available (attributable) to common shareholder	$ (195)	$ 143	$ (78)	$ (1,109)	$ (861)	$ 98	$ 26	$ 124