Derivatives and Risk Management - Schedule of Designated Hedge Gain (Loss) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion reclassified from AOCI to earnings	$ 127	$ 135	$ (15)
Effective portion recognized in other comprehensive income (loss)	4	161	211
Fuel hedge contract
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion reclassified from AOCI to earnings	0	0	15
Effective portion recognized in other comprehensive income (loss)	0	0	(15)
Interest rate contract
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion reclassified from AOCI to earnings	(31)	0	(5)
Effective portion recognized in other comprehensive income (loss)	38	28	14
Foreign exchange contract
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion reclassified from AOCI to earnings	158	135	(25)
Effective portion recognized in other comprehensive income (loss)	$ (34)	$ 133	$ 212