Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives (effective portion)		$ 32	$ (120)
(Gain) loss reclassified from AOCI into income (effective portion)	[1]	356	622
(Gain) loss recognized in income on derivatives (ineffective portion)	[2]	32	(14)
Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		71	12
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives (effective portion)	[3]	32	(122)
(Gain) loss reclassified from AOCI into income (effective portion)	[1],[3]	349	613
(Gain) loss recognized in income on derivatives (ineffective portion)	[2]	31	(11)
Fuel derivatives | Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		75	14
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in AOCI on derivatives (effective portion)	[3]	0	2
(Gain) loss reclassified from AOCI into income (effective portion)	[1],[3]	7	9
(Gain) loss recognized in income on derivatives (ineffective portion)	[2]	1	(3)
Interest rate derivatives | Not Designated as Hedging Instrument | Other Nonoperating Income (Expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		$ (4)	$ (2)

[1] Amounts related to fuel derivative contracts and interest rate derivatives are included in Fuel and oil and Interest expense, respectively.

[2] Amounts are included in Other (gains) losses, net.

[3] Net of tax