Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)		$ 55	$ (114)	$ 134	$ (74)
(Gain) Loss reclassified from AOCI into income (effective portion)	[1]	102	152	192	349
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	8	(4)	21	0
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	[3]	54	(116)	133	(80)
(Gain) Loss reclassified from AOCI into income (effective portion)	[1],[3]	100	149	188	344
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	8	(3)	21	1
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	[3]	1	2	1	6
(Gain) Loss reclassified from AOCI into income (effective portion)	[1],[3]	2	3	4	5
(Gain) Loss recognized in income on derivatives (ineffective portion)	[2]	0	(1)	0	(1)
Other Nonoperating Income Expense | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		31	(88)	82	(12)
Other Nonoperating Income Expense | Fuel derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		32	(88)	84	(12)
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	$ (2)	$ 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