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SHARE-BASED COMPENSATION - Information and Valuation Assumptions (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 12 Months Ended
Oct. 31, 2014
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Mar. 31, 2018
Share-based compensation          
Total compensation expense   $ 7,895 $ 6,628 $ 5,073  
Assumptions used to determine the valuation of granted options          
Expected dividend yield (as a percent)   0.00% 0.00% 0.00%  
Expected term (in years)       6 years 3 months  
Additional disclosures          
Total compensation cost related to non-vested options not yet recognized   $ 15,677      
Weighted average period to recognize compensation cost related to non-vested options not yet recognized   2 years 5 months 9 days      
2007 Stock Option Plan          
Share-based compensation          
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter   25.00%      
Number of anniversary dates upon which options become exercisable   3 years      
Maximum term of stock option plan   10 years      
Minimum          
Assumptions used to determine the valuation of granted options          
Exercise price   $ 20.52 $ 14.36 $ 14.40  
Expected volatility (as a percent)   36.06% 33.44% 23.90%  
Risk-free rate for expected term (as a percent)   2.33% 1.88% 1.23%  
Expected term (in years)   6 years 5 years    
Maximum          
Assumptions used to determine the valuation of granted options          
Exercise price   $ 24.29 $ 20.87 $ 36.60  
Expected volatility (as a percent)   38.36% 36.38% 24.76%  
Risk-free rate for expected term (as a percent)   2.84% 2.34% 2.06%  
Expected term (in years)   6 years 3 months 6 years 3 months    
Service-based awards | 2014 Plan          
Share-based compensation          
Maximum term of stock option plan   10 years 10 years    
Service-based awards | 2014 Plan | Tranche One          
Share-based compensation          
Granted (in shares)   105,000      
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter 25.00%        
Number of anniversary dates upon which options become exercisable   3 years      
Service-based awards | 2014 Plan | Tranche Two          
Share-based compensation          
Granted (in shares)   225,135      
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter   33.30%      
Number of anniversary dates upon which options become exercisable   2 years      
Service-based awards | 2014 Plan | Key employees          
Additional disclosures          
Number of shares issued during the period (in shares)   330,135 2,805,976 1,165,000  
Vested and issued (in shares)     (200,000)    
Make-whole inducement award          
Share-based compensation          
Number of anniversary dates upon which options become exercisable   2 years      
Maximum term of stock option plan   10 years      
Make-whole inducement award | Executive          
Share-based compensation          
Granted (in shares)   81,351      
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter   33.30%      
Stock Options          
Share-based compensation          
Granted (in shares)   411,486 4,066,735 1,546,532  
Weighted average grant-date fair value of options granted (in dollars per share)   $ 8.75 $ 6.23 $ 6.34  
Additional disclosures          
Intrinsic value of options exercised   $ 4,560 $ 11,973 $ 13,048  
Stock Options | Key employees          
Share-based compensation          
Maximum term of stock option plan   10 years      
Assumptions used to determine the valuation of granted options          
Expected dividend yield (as a percent)   0.00%      
Stock Options | Key employees | Tranche One          
Share-based compensation          
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter   25.00%      
Number of anniversary dates upon which options become exercisable   3 years      
Stock Options | Key employees | Tranche Two          
Share-based compensation          
Percentage of options exercisable in installments beginning on the first anniversary of the grant date and each of the three anniversaries thereafter   33.00%      
Number of anniversary dates upon which options become exercisable   2 years      
Performance-based RSUs | 2014 Plan          
Share-based compensation          
Maximum term of stock option plan   10 years      
Additional disclosures          
Incremental compensation expense     $ 0 $ 0  
Performance-based RSUs | 2014 Plan | Key employees          
Share-based compensation          
Granted (in shares)   800,000      
Number of shares options vested at time of grant (in shares)         225,862
Additional disclosures          
Number of shares issued during the period (in shares)     260,759 381,532