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LONG-TERM DEBT LONG-TERM DEBT - SECURED DEBT (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
aircraft
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]      
Debt instrument, increase (decrease) $ (395)    
Secured debt, number of aircrafts securing debt financing | aircraft 113    
Long-term Debt $ 2,569 $ 2,964  
Secured Debt, Number of Spare Engines Securing Debt Financing | aircraft 2    
Long-term debt payments $ 397 $ 249 $ 116
Repayments of debt 74    
Secured Debt [Member] | Secured Debt, Aircraft Collateral [Member]      
Debt Instrument [Line Items]      
Long-term Debt 2,200    
Secured Debt [Member] | Secured Debt, Aircraft Purchase Agreements [Member]      
Debt Instrument [Line Items]      
Long-term Debt $ 392