Selected Quarterly Financial Data (Unaudited) - Summary of 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	[1]	Dec. 31, 2017	[1]
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 10,888	$ 11,380	$ 11,402	$ 9,589	$ 10,491	$ 11,003	$ 10,777	$ 9,032	$ 43,259	$ 41,303		$ 37,784
Income from operations	861	1,473	1,472	495	635	1,187	1,145	262	4,301	3,229		3,618
Net income	$ 641	$ 1,024	$ 1,052	$ 292	$ 461	$ 833	$ 683	$ 145	$ 3,009	$ 2,122	[2],[3],[4]	$ 2,143	[2],[3],[4]
Earnings per share, basic (in dollars per share)	$ 2.54	$ 4.01	$ 4.03	$ 1.09	$ 1.70	$ 3.06	$ 2.48	$ 0.51	$ 11.63	$ 7.70		$ 7.08
Earnings per share, diluted (in dollars per share)	$ 2.53	$ 3.99	$ 4.02	$ 1.09	$ 1.69	$ 3.05	$ 2.48	$ 0.51	$ 11.58	$ 7.67		$ 7.06

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8

[3] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[4] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.