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COMMITMENTS AND CONTINGENCIES - Operating lease commitments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
COMMITMENTS AND CONTINGENCIES      
2018 $ 9,971    
2019 8,902    
2020 7,807    
2021 7,323    
2022 and thereafter 6,577    
Total 40,580    
Rental expense for all operating leases $ 11,760 $ 10,580 $ 9,432