Quarterly Financial Data (Unaudited) - Summarized Results of Operations (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 Data
Operating revenue	$ 10,742	$ 11,953	$ 11,775	$ 9,968	$ 10,229	$ 11,061	$ 10,747	$ 9,101
Operating income	1,090	1,645	1,684	844	1,162	1,823	1,982	999	$ 5,264	$ 5,966	$ 6,996
Net Income	$ 1,019	$ 1,322	$ 1,036	$ 557	$ 299	$ 1,159	$ 1,186	$ 561	$ 3,935	$ 3,205	$ 4,195
Basic earnings per share (usd per share)	$ 1.50	$ 1.93	$ 1.49	$ 0.79	$ 0.42	$ 1.62	$ 1.63	$ 0.77	$ 5.69	$ 4.45	$ 5.59
Diluted earnings per share (usd per share)	$ 1.49	$ 1.92	$ 1.49	$ 0.79	$ 0.42	$ 1.61	$ 1.62	$ 0.77	$ 5.67	$ 4.43	$ 5.55
ASU No. 2016-02 | As Previously Reported
Quarterly Financial Data
Operating revenue		$ 11,953	$ 11,775	$ 9,968
Operating income		1,642	1,680	840
Net Income		$ 1,312	$ 1,025	$ 547
Basic earnings per share (usd per share)		$ 1.91	$ 1.47	$ 0.78
Diluted earnings per share (usd per share)		$ 1.91	$ 1.47	$ 0.77