Revenue - Narrative (Details) - USD ($) $ in Millions			3 Months Ended		6 Months Ended
	Apr. 01, 2020	Mar. 31, 2020	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue From Contract With Customer [Line Items]
Operating revenue			$ 1,475	$ 11,402	$ 9,454	$ 20,991
Advance Ticket Sales
Revenue From Contract With Customer [Line Items]
Expiration period for ETCs	24 months	12 months
Expiration period for FFCs	24 months
ETC and FFC liabilities			$ 3,000		$ 3,000
Percent of ETCs and FFCs with expiration dates extending beyond 12 months			93.00%		93.00%
Revenue recognized			$ 500	3,900	$ 2,800	3,200
Ancillary Fees
Revenue From Contract With Customer [Line Items]
Operating revenue			66	636	542	1,200
Other Operating Revenue
Revenue From Contract With Customer [Line Items]
Operating revenue			392	621	1,042	1,199
Other Operating Revenue | Chase and Other Partner Agreements
Revenue From Contract With Customer [Line Items]
Operating revenue			$ 339	$ 499	$ 869	$ 972