Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)		$ (29)	$ 17	$ 105	$ (58)
(Gain) Loss reclassified from AOCI into income (effective portion)	[1]	96	143	288	491
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	8	(6)	29	(6)
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	[3]	(29)	19	104	(62)
(Gain) Loss reclassified from AOCI into income (effective portion)	[1],[3]	94	141	282	484
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	8	(4)	29	(3)
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	[3]	0	(2)	1	4
(Gain) Loss reclassified from AOCI into income (effective portion)	[1],[3]	2	2	6	7
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	0	(2)	0	(3)
Other Nonoperating Income Expense | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		(5)	35	77	23
Other Nonoperating Income Expense | Fuel derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		(4)	35	80	23
Other Nonoperating Income Expense | Interest rate derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		$ (1)	$ 0	$ (3)	$ 0

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

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

[3] Net of tax