Supplemental Quarterly Results (Unaudited) (Schedule of Quarterly Financial Inform) (Details) - USD ($) $ / shares in Units, $ 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 Data [Abstract]
Operating revenues	$ 779	$ 527	$ 410	$ 592	$ 745	$ 636	$ 667	$ 990	$ 2,308	$ 3,038	$ 3,862
Operating loss	(244)	(381)	(756)	(1,490)	64	(29)	22	213	(2,871)	270	797
Net (loss) income	$ (92)	$ (593)	$ (880)	$ (1,547)	$ 110	$ 49	$ 138	$ 594	$ (3,112)	$ 891	$ 535
(Loss) earnings per share - basic (in dollars per share)	$ (0.14)	$ (1.04)	$ (1.63)	$ (2.86)	$ 0.20	$ 0.09	$ 0.26	$ 1.10	$ (5.42)	$ 1.65	$ 0.93
(Loss) earnings per share - diluted (in dollars per share)	$ (0.14)	$ (1.04)	$ (1.63)	$ (2.86)	$ 0.20	$ 0.09	$ 0.26	$ 1.10	$ (5.42)	$ 1.65	$ 0.93