Segments & Geographic Information - Geographic Information (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues
Operating revenue	$ 9,502	$ 11,107	$ 10,707	$ 9,388	$ 9,647	$ 11,178	$ 10,621	$ 8,916	$ 40,704	$ 40,362	$ 37,773
Domestic | Operating Segments
Revenues
Operating revenue									27,884	26,898	24,857
Atlantic | Operating Segments
Revenues
Operating revenue									6,505	6,757	6,446
Pacific | Operating Segments
Revenues
Operating revenue									3,503	3,948	4,086
Latin America | Operating Segments
Revenues
Operating revenue									$ 2,812	$ 2,759	$ 2,384