Revenue - Passenger Revenue Breakout (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of Revenue [Line Items]
Passenger Revenue	$ 5,358	$ 5,379	$ 9,943	$ 9,925
Passenger Revenue Non Loyalty
Disaggregation of Revenue [Line Items]
Passenger Revenue	4,605	4,643	8,552	8,551
Passenger Loyalty Air Transportation
Disaggregation of Revenue [Line Items]
Passenger Revenue	588	602	1,079	1,108
Passenger Ancillary Sold Separately
Disaggregation of Revenue [Line Items]
Passenger Revenue	$ 165	$ 134	$ 312	$ 266