Revenue - Revenue by Principal Geographic Region (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenue	$ 9,589	$ 9,032	[1]
Domestic (U.S. and Canada)
Disaggregation of Revenue [Line Items]
Revenue	5,875	5,476
Atlantic
Disaggregation of Revenue [Line Items]
Revenue	1,458	1,389
Pacific
Disaggregation of Revenue [Line Items]
Revenue	1,281	1,231
Latin America
Disaggregation of Revenue [Line Items]
Revenue	$ 975	$ 936

[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 I, Item 1 of this report for additional information.