Significant Accounting Policies - Roll Forward of Frequent Flier Deferred Revenue (Details) - Transportation Advance Ticket Arrangement - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Movement in Deferred Revenue [Roll Forward]
Total Frequent flyer deferred revenue - beginning balance	$ 4,783	$ 4,889
Total miles awarded	2,451	2,077
Travel miles redeemed (Passenger revenue)	(2,068)	(2,004)
Non-travel miles redeemed (Other operating revenue)	(161)	(179)
Total Frequent flyer deferred revenue - ending balance	$ 5,005	$ 4,783