Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018
Revenue From Contract With Customer [Line Items]
Operating revenue	$ 11,402	$ 10,777	[1]	$ 20,991	$ 19,809	[1]
Advance Ticket Sales
Revenue From Contract With Customer [Line Items]
Revenue recognized	3,900	3,500		3,200	2,900
Ancillary Fees
Revenue From Contract With Customer [Line Items]
Operating revenue	636	555		1,200	1,100
Frequent Flyer
Revenue From Contract With Customer [Line Items]
Operating revenue	$ 499	$ 480		$ 972	$ 974

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.