Selected Quarterly Financial Data (Unaudited) (Details) - USD ($) $ / shares in Units, $ 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	$ 3,469	$ 3,479	$ 3,609	$ 3,552	$ 3,580	$ 3,613	$ 3,831	$ 3,744	$ 14,109	$ 14,768	$ 14,314
Cost of revenue	2,022	2,007	2,099	2,059	2,096	2,096	2,231	2,181	8,187	8,604	8,306
Operating income	824	868	871	839	860	889	932	903	3,402	3,584	3,485
Net income	$ 641	$ 660	$ 623	$ 597	$ 607	$ 638	$ 666	$ 652	$ 2,521	$ 2,563	$ 1,687
Net Income Per Share:
Basic (in dollars per share)	$ 2.00	$ 2.05	$ 1.92	$ 1.82	$ 1.84	$ 1.91	$ 1.98	$ 1.92	$ 7.78	$ 7.65	$ 4.90
Diluted (in dollars per share)	$ 1.99	$ 2.04	$ 1.91	$ 1.81	$ 1.83	$ 1.90	$ 1.97	$ 1.90	$ 7.74	$ 7.60	$ 4.86