Significant Accounting Policies - Operating Revenue by Principal Geographic Region (Details) - USD ($) $ 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
Disaggregation of Revenue [Line Items]
Revenue	$ 10,491	$ 11,003	$ 10,777	$ 9,032	$ 9,451	$ 9,899	$ 10,008	$ 8,426	$ 41,303	$ 37,784	[1]	$ 36,558	[1]
Domestic (U.S. and Canada)
Disaggregation of Revenue [Line Items]
Revenue									25,552	23,114		22,151
Atlantic
Disaggregation of Revenue [Line Items]
Revenue									7,103	6,340		6,194
Pacific
Disaggregation of Revenue [Line Items]
Revenue									5,188	4,914		4,984
Latin America
Disaggregation of Revenue [Line Items]
Revenue									$ 3,460	$ 3,416		$ 3,229

[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.