Segments and Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Operating revenue	$ 10,245	$ 11,060	$ 10,791	$ 9,148	$ 9,458	$ 10,483	$ 10,447	$ 9,251	$ 41,244	$ 39,639	$ 40,704
Domestic
Revenues
Operating revenue									29,556	28,108	27,884
Atlantic
Revenues
Operating revenue									6,044	5,919	6,505
Pacific
Revenues
Operating revenue									2,730	2,939	3,503
Latin America
Revenues
Operating revenue									$ 2,914	$ 2,673	$ 2,812