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COMMITMENTS AND CONTINGENCIES - Lease commitments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Lease commitments      
Rental expenses $ 19,765 $ 17,116 $ 15,451
Future minimum lease payments under non-cancelable operating lease agreements      
2018 10,230    
2019 8,884    
2020 8,025    
2021 6,889    
2022 4,965    
Thereafter 71,667    
Total $ 110,660    
Minimum      
Lease commitments      
Remaining terms for various operating lease agreements 1 month    
Maximum      
Lease commitments      
Remaining terms for various operating lease agreements 402 months