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DERIVATIVE INSTRUMENTS - FAIR VALUE OF HEDGE POSITIONS (DETAILS) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative [Line Items]      
Net cash received (paid) to for new positions and settlements $ (9)us-gaap_PaymentsForProceedsFromHedgeInvestingActivities $ 5us-gaap_PaymentsForProceedsFromHedgeInvestingActivities $ (19)us-gaap_PaymentsForProceedsFromHedgeInvestingActivities
B737-800 [Member]      
Derivative [Line Items]      
Capital Leased Assets, Number of B737-800 Aircraft 6us-gaap_CapitalLeasedAssetsNumberOfUnits
/ us-gaap_AircraftTypeAxis
= us-gaap_B737800Member		    
Not Designated as Hedging Instrument [Member] | Fuel hedge contracts [Member]      
Derivative [Line Items]      
Fuel hedge contracts outstanding (in gallons) 246,000,000invest_DerivativeNonmonetaryNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
/ us-gaap_HedgingDesignationAxis
= us-gaap_NondesignatedMember