Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives		$ (79)	$ 79
(Gain) Loss recognized in income on derivatives (ineffective portion)	[1]	0	14
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[2]	(78)	79
(Gain) Loss recognized in income on derivatives (ineffective portion)	[1]	0	14
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives	[2]	(1)	0
(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		(1)	50
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	51
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)

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

[2] Net of tax