Quarterly Financial Information (Schedule of Quarterly Financial Information) (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]
Operating revenue	$ 688	$ 755	$ 624	$ 672	$ 771	$ 795	$ 698	$ (335)
Operating income (loss)	136	180	144	171	(76)	261	17	(1,143)	$ 631	$ (941)	$ (831)
Total comprehensive income (loss)	43	106	67	93	(191)	143	(70)	(1,103)	$ 308	$ (1,221)
Comprehensive income (loss) available (attributable) to common shareholder	$ 39	$ 101	$ 69	$ 88	$ (195)	$ 143	$ (78)	$ (1,109)