Quarterly Financial Data (Unaudited) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating quarterly financial data
Operating revenues	$ 2,114	$ 2,141	$ 2,133	$ 2,162	$ 2,208	$ 2,226	$ 2,223	$ 2,253	$ 8,550	$ 8,910	$ 8,964
Operating income	600	560	573	580	521	576	602	625	2,313	2,324	2,260
Income tax expense (benefit)	(378)	168	170	174	152	159	179	188	134	678	659
Net income	846	$ 265	$ 268	$ 278	238	$ 255	$ 288	$ 304	$ 1,657	$ 1,085	$ 1,074
Provisional tax benefit, amount	$ 555
Federal statutory income tax rate (as a percent)	35.00%								35.00%	35.00%	35.00%
Federal income tax rate after enactment of Tax Reform (as a percent)	21.00%
Severance costs					$ 78