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SHARE-BASED COMPENSATION - RSUs to Employees (Details) - RSUs - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
SHARE-BASED COMPENSATION      
Shares granted to employees 1,033,001 868,036 574,488
Total compensation cost at the date of grant $ 13,300 $ 12,100 $ 13,200
Total recognized compensation cost 9,314 $ 7,713 $ 5,611
Total unrecognized share-based compensation costs $ 21,867    
Weighted-average period of recognition of compensation expense 2 years 7 months 24 days    
Number of Shares      
Unvested at the beginning of the period (in shares) 1,402,859    
Granted (in shares) 1,033,001 868,036 574,488
Vested (in shares) 569,423    
Forfeited (in shares) (134,390)    
Unvested at the end of the period (in shares) 1,732,047 1,402,859  
Weighted Average Grant-Date Fair Value      
Unvested at the beginning of the period (in dollars per share) $ 17.27    
Granted (in dollars per share) 13.11    
Vested (in dollars per share) 14.49    
Forfeited (in dollars per share) 20.89    
Unvested at the end of the period (in dollars per share) $ 15.42 $ 17.27  
Total fair value of shares vested $ 12,091 $ 7,271 $ 4,641
Minimum      
SHARE-BASED COMPENSATION      
Vesting period 1 year    
Maximum      
SHARE-BASED COMPENSATION      
Vesting period 4 years