Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 2,113	$ 2,149	$ 2,101	$ 2,130	$ 2,114	$ 2,141	$ 2,133	$ 2,162	$ 8,493	$ 8,550	$ 8,910
Operating income	685	717	626	632	600	560	573	580	2,660	2,313	2,324
Income tax expense	172	111	81	130	(378)	168	170	174	494	134	678
Net income	$ 405	$ 453	$ 427	$ 380	$ 846	$ 265	$ 268	$ 278	$ 1,665	1,657	$ 1,085
Provisional tax benefit, amount										$ 555
Federal statutory income tax rate (as a percent)					35.00%				21.00%	35.00%	35.00%
Federal income tax rate after enactment of Tax Reform (as a percent)									21.00%