Quarterly Financial Data (Unaudited) (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	$ 2,012	$ 2,039	$ 2,051	$ 2,055	$ 2,113	$ 2,149	$ 2,101	$ 2,130	$ 8,157	$ 8,493	$ 8,550
Operating income	626	748	750	760	685	717	626	632	2,884	2,660	2,313
Net income	$ 386	$ 477	$ 477	$ 487	$ 405	$ 453	$ 427	$ 380	$ 1,827	$ 1,665	$ 1,657