Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenue From Contract With Customer [Line Items]
Operating revenue	$ 3,221	$ 7,979
Advance Ticket Sales
Revenue From Contract With Customer [Line Items]
Revenue recognized	700	2,600
ETCs and FFCs
Revenue From Contract With Customer [Line Items]
Advance ticket sales liability	2,800
Ancillary Fees
Revenue From Contract With Customer [Line Items]
Operating revenue	308	705
Other Operating Revenue
Revenue From Contract With Customer [Line Items]
Operating revenue	408	650
Other Operating Revenue | Chase and Other Partner Agreements
Revenue From Contract With Customer [Line Items]
Operating revenue	$ 362	$ 530