Significant Accounting Policies - Operating Revenue by Principal Geographic Region (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenue	$ 3,412	$ 2,489	$ 1,475	$ 7,979	$ 10,888	$ 11,380	$ 11,402	$ 9,589	$ 15,355	$ 43,259	$ 41,303
Domestic (U.S. and Canada)
Disaggregation of Revenue [Line Items]
Operating revenue									9,911	26,960	25,552
Atlantic
Disaggregation of Revenue [Line Items]
Operating revenue									2,226	7,387	7,103
Pacific
Disaggregation of Revenue [Line Items]
Operating revenue									1,706	5,132	5,188
Latin America
Disaggregation of Revenue [Line Items]
Operating revenue									$ 1,512	$ 3,780	$ 3,460