Quarterly report pursuant to Section 13 or 15(d)

REVENUES - Operating Revenue (Details)

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REVENUES - Operating Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Disaggregation of Revenue [Line Items]        
Revenues $ 54,610 $ 61,385 $ 169,660 $ 173,790
Dry-leasing | United States        
Disaggregation of Revenue [Line Items]        
Revenues 1,142 486 2,984 1,015
Dry-leasing | Foreign        
Disaggregation of Revenue [Line Items]        
Revenues 1,574 2,146 5,560 11,698
Operating Income        
Disaggregation of Revenue [Line Items]        
Revenues 51,894 58,753 161,116 161,077
Operating Income | United States        
Disaggregation of Revenue [Line Items]        
Revenues 38,229 41,989 117,673 112,862
Operating Income | Foreign        
Disaggregation of Revenue [Line Items]        
Revenues 13,665 16,764 43,443 48,215
Operating Income | Oil and Gas        
Disaggregation of Revenue [Line Items]        
Revenues 49,138 53,331 153,221 147,129
Operating Income | Oil and Gas | United States        
Disaggregation of Revenue [Line Items]        
Revenues 35,473 36,567 109,778 98,914
Operating Income | Oil and Gas | Foreign        
Disaggregation of Revenue [Line Items]        
Revenues 13,665 16,764 43,443 48,215
Operating Income | Emergency response services        
Disaggregation of Revenue [Line Items]        
Revenues 2,756 2,487 7,895 8,877
Operating Income | Flightseeing        
Disaggregation of Revenue [Line Items]        
Revenues $ 0 $ 2,935 $ 0 $ 5,071