Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in AOCI on derivatives		$ (56)	$ (79)
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in AOCI on derivatives	[1]	(68)	(78)
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in AOCI on derivatives	[1]	12	(1)
Interest rate derivatives | Interest rate derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
Gain recognized in income on derivatives		$ 0	$ (1)

[1] Net of tax