Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended								12 Months Ended
	Oct. 31, 2018	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		$ 5,570	$ 5,606	$ 5,578	$ 5,647	$ 5,778	$ 5,818	$ 5,902	$ 5,945	$ 22,401	$ 23,443	$ 17,656
Operating (loss) income		847	950	976	(5,499)	(1,841)	894	767	750	(2,726)	570	2,009
Net (loss) income		$ 223	$ 302	$ 371	$ (6,165)	$ (2,412)	$ 272	$ 292	$ 115	$ (5,269)	$ (1,733)	$ 1,389
Basic (loss) earnings per common share (in dollars per share)		$ 0.21	$ 0.28	$ 0.35	$ (5.77)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ (4.92)	$ (1.63)	$ 2.21
Diluted (loss) earnings per common share (in dollars per share)		$ 0.21	$ 0.28	$ 0.35	$ (5.77)	$ (2.26)	$ 0.25	$ 0.27	$ 0.11	$ (4.92)	$ (1.63)	$ 2.21
Goodwill impairment	$ 2,700				$ 6,500	$ 2,700			$ 2,700	$ 6,506	$ 2,726	$ 0
Acquisition related costs		$ 123	$ 38	$ 39	$ 34	$ 117	$ 43	$ 162	$ 71