Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018		Jun. 30, 2017
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives		$ (229)	$ 55	$ 308	[1],[2]	$ 134
(Gain) Loss recognized in income on derivatives (ineffective portion)	[3]	0	8	0		21
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[4]	(228)	54	(306)		133
(Gain) Loss recognized in income on derivatives (ineffective portion)	[3]	0	8	0		21
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[4]	(1)	1	(2)		1
(Gain) Loss recognized in income on derivatives (ineffective portion)	[3]	0	0	0		0
Other Nonoperating Income Expense | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		(1)	31	(2)		82
Other Nonoperating Income Expense | Fuel derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		0	32	0		84
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)	$ (1)	$ (2)		$ (2)

[1] The Company has the option of providing cash, letters of credit, or pledging aircraft as collateral.

[2] Thresholds may vary based on changes in credit ratings within investment grade.

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

[4] Net of tax