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STOCK-BASED COMPENSATION - Fair Value and Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Aggregate Intrinsic Value      
Weighted average period for recognition of cost 2 years 9 months 18 days    
Minimum      
Valuation assumptions      
Expected term (in years) 6 years 3 months 10 days 5 years 5 months 12 days  
Maximum      
Valuation assumptions      
Expected term (in years)   6 years 5 months 4 days  
Option Plan Shares      
Valuation assumptions      
Risk-free interest rates, minimum (as a percent) 2.54% 2.73%  
Risk-free interest rates, maximum (as a percent) 2.59% 2.79%  
Expected term (in years) 6 years 3 months 10 days    
Expected volatility, minimum (as a percent) 64.95% 61.82%  
Expected volatility, maximum (as a percent) 64.99% 66.17%  
Dividend yield (as a percent) 0.00% 0.00%  
Weighted average fair value per share of options granted during the period (in dollars per share) $ 57.11 $ 24.55  
ESPP Shares      
Valuation assumptions      
Risk-free interest rates, minimum (as a percent) 2.31% 2.05%  
Risk-free interest rates, maximum (as a percent) 2.44% 2.50%  
Expected volatility, minimum (as a percent) 55.00% 51.75%  
Expected volatility, maximum (as a percent) 57.97% 65.39%  
Dividend yield (as a percent) 0.00% 0.00%  
Weighted average fair value per share of options granted during the period (in dollars per share) $ 35.91 $ 18.68  
ESPP Shares | Minimum      
Valuation assumptions      
Expected term (in years) 6 months 6 months  
ESPP Shares | Maximum      
Valuation assumptions      
Expected term (in years) 2 years 2 years  
Restricted Shares and RSUs      
Restricted Shares and RSUs      
Outstanding at the beginning of the period (in shares) 6,246,174    
Granted (in shares) 1,672,854    
Released (in shares) (3,808,767)    
Forfeited (in shares) (207,978)    
Outstanding at the end of the period (in shares) 3,902,283   6,246,174
Weighted Average Grant Date Fair Value      
Outstanding at the beginning of the period (in dollars per share) $ 23.16    
Granted (in dollars per share) 94.07    
Released (in dollars per share) 14.80    
Forfeited (in dollars per share) 55.96    
Outstanding at the end of the period (in dollars per share) $ 60.37   $ 23.16
Stock Plans      
Shares      
Outstanding at the beginning of the period (in shares) 2,531,561    
Granted (in shares) 186,044    
Exercised (in shares) (492,699)    
Forfeited (in shares) (25,792)    
Outstanding at the end of the period (in shares) 2,199,114   2,531,561
Exercisable at the end of the period (in shares) 1,252,387    
Weighted Average Exercise Price      
Outstanding at the beginning of the period (in dollars per share) $ 17.86    
Granted (in dollars per share) 92.61    
Exercised (in dollars per share) 12.97    
Forfeited (in dollars per share) 33.25    
Outstanding at the end of the period (in dollars per share) 25.10   $ 17.86
Exercisable at the end of the period (in dollars per share) $ 15.28    
Weighted Average Remaining Contractual Term      
Outstanding at the end of the period 6 years 4 months 24 days   6 years 7 months 6 days
Exercisable at the end of the period 5 years 4 months 24 days    
Aggregate Intrinsic Value      
Outstanding at the end of the period $ 143,943    
Exercisable at the end of the period 94,038    
Total intrinsic value of options exercised 41,700 $ 40,100  
Unrecognized compensation cost $ 188,400    
Stock Plans | Maximum      
Aggregate Intrinsic Value      
Market price (in dollars per share) $ 90.37    
Omnibus Long Term Incentive Plan2010 | Option Plan Shares      
Aggregate Intrinsic Value      
Grant options 0