Quarterly report pursuant to Section 13 or 15(d)

VARIABLE INTEREST ENTITIES - Narrative (Details)

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VARIABLE INTEREST ENTITIES - Narrative (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Dec. 31, 2011
Variable Interest Entity [Line Items]            
Operating revenues $ 61,385 $ 65,006 $ 173,790 $ 190,939    
Aeroleo            
Variable Interest Entity [Line Items]            
Economic interest percent           50.00%
Voting interest percent           20.00%
Aeroleo | Variable Interest Entity, Primary Beneficiary            
Variable Interest Entity [Line Items]            
Assets 11,800   11,800   $ 12,900  
Liabilities 9,200   9,200   $ 9,200  
Operating revenues 9,300 7,500 26,500 21,600    
Net gain (loss) from consolidation $ 400 $ (2,400) $ (1,300) $ (4,500)