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, 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	[1]	Dec. 31, 2016	[1]
Quarterly Financial Information Disclosure [Abstract]
Operating revenue	$ 10,491	$ 11,003	$ 10,777	$ 9,032	$ 9,451	$ 9,899	$ 10,008	$ 8,426	$ 41,303	$ 37,784		$ 36,558
Operating income	652	1,203	1,161	276	776	1,138	1,437	320	3,292	3,671		4,344
Net income	$ 462	$ 836	$ 684	$ 147	$ 579	$ 645	$ 821	$ 99	$ 2,129	$ 2,144	[2],[3],[4]	$ 2,234	[2],[3],[4]
Earnings per share, basic (in dollars per share)	$ 1.71	$ 3.07	$ 2.49	$ 0.52	$ 1.99	$ 2.15	$ 2.67	$ 0.32	$ 7.73	$ 7.08		$ 6.77
Earnings per share, diluted (in dollars per share)	$ 1.70	$ 3.06	$ 2.48	$ 0.52	$ 1.98	$ 2.15	$ 2.67	$ 0.32	$ 7.70	$ 7.06		$ 6.76

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. 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. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[3] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). 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. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.