Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives				$ 430	$ 105
(Gain) Loss recognized in income on derivatives (ineffective portion)	[1]			0	29
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[2]	$ (122)	$ (29)	(428)	104
(Gain) Loss recognized in income on derivatives (ineffective portion)	[1]	0	8	0	29
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[2]			(2)	1
(Gain) Loss recognized in income on derivatives (ineffective portion)	[1]			0	0
Other Nonoperating Income Expense | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives		0	(5)	(2)	77
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	(4)	0	80
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		$ 0	$ (1)	$ (2)	$ (3)

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

[2] Net of tax