Revenue - Operating Revenue by Geographic Region (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenue	$ 5,471	$ 1,475	$ 8,692	$ 9,454
Domestic (U.S. and Canada)
Disaggregation of Revenue [Line Items]
Operating revenue	3,767	925	5,878	6,003
Atlantic
Disaggregation of Revenue [Line Items]
Operating revenue	585	219	995	1,434
Pacific
Disaggregation of Revenue [Line Items]
Operating revenue	392	258	700	1,064
Latin America
Disaggregation of Revenue [Line Items]
Operating revenue	$ 727	$ 73	$ 1,119	$ 953