Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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	$ 3,580	$ 3,613	$ 3,831	$ 3,744	$ 3,629	$ 3,615	$ 3,599	$ 3,471	$ 14,768	$ 14,314	$ 13,599
Cost of revenue	2,096	2,096	2,231	2,181	2,124	2,092	2,087	2,003	8,604	8,306	7,908
Operating income	860	889	932	903	846	960	872	807	3,584	3,485	3,056
Net income	$ 607	$ 638	$ 666	$ 652	$ (76)	$ 640	$ 587	$ 536	$ 2,563	$ 1,687	$ 2,035
Net Income Per Share:
Basic (in dollars per share)	$ 1.84	$ 1.91	$ 1.98	$ 1.92	$ (0.22)	$ 1.86	$ 1.70	$ 1.55	$ 7.65	$ 4.90	$ 5.73
Diluted (in dollars per share)	$ 1.83	$ 1.90	$ 1.97	$ 1.90	$ (0.22)	$ 1.85	$ 1.69	$ 1.54	$ 7.60	$ 4.86	$ 5.70
Income tax expense (benefit)					$ 658					$ 658
Tax, earnings per share, diluted (in dollars per share)					$ 1.92
Litigation settlement, amount awarded from other party							$ 95
Litigation settlement, payment period, maximum							120 days
Gain (loss) related to litigation settlement						$ 80	$ 15		$ 0	$ 95	$ 0