Financial Derivative Instruments - (Gain) Loss by Hedging Relationship (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	$ 212		$ (103)		$ 106		$ 101
(Gain) Loss reclassified from AOCI into income (effective portion)	8	[1]	28	[1]	13	[1]	102	[1]
(Gain) Loss recognized in income on derivatives (ineffective portion)	9	[2]	15	[2]	(34)	[2]	26	[2]
Fuel derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	214	[3]	(105)	[3]	104	[3]	113	[3]
(Gain) Loss reclassified from AOCI into income (effective portion)	4	[1],[3]	24	[1],[3]	3	[1],[3]	88	[1],[3]
(Gain) Loss recognized in income on derivatives (ineffective portion)	11	[2]	15	[2]	(31)	[2]	27	[2]
Interest rate derivatives | Cash Flow Hedging
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) Loss recognized in AOCI on derivatives (effective portion)	(2)	[3]	2	[3]	2	[3]	(12)	[3]
(Gain) Loss reclassified from AOCI into income (effective portion)	4	[1],[3]	4	[1],[3]	10	[1],[3]	14	[1],[3]
(Gain) Loss recognized in income on derivatives (ineffective portion)	(2)	[2]	0	[2]	(3)	[2]	(1)	[2]
Other Nonoperating Income Expense | Fuel derivatives | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss) [Line Items]
(Gain) loss recognized in income on derivatives	$ 39		$ (93)		$ (1)		$ (122)

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

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

[3] Net of tax