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STOCK-BASED COMPENSATION - Fair Value and Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Jun. 28, 2019
Valuation assumptions            
Dividend yield (as a percent) 0.00%          
Aggregate Intrinsic Value            
Weighted average period for recognition of cost     3 years      
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 8 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)     65.00% 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% 2.31% 2.05%    
Risk-free interest rates, maximum (as a percent) 2.44% 2.50% 2.44% 2.50%    
Expected volatility, minimum (as a percent) 55.00% 51.70% 55.00% 51.70%    
Expected volatility, maximum (as a percent) 57.60% 65.40% 57.60% 65.40%    
Dividend yield (as a percent)   0.00% 0.00% 0.00%    
Weighted average fair value per share of options granted during the period (in dollars per share) $ 35.91 $ 18.68 $ 35.91 $ 18.68    
ESPP Shares | Minimum            
Valuation assumptions            
Expected term (in years) 6 months 6 months 6 months 6 months    
ESPP Shares | Maximum            
Valuation assumptions            
Expected term (in years) 2 years 2 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)     1,513,290      
Released (in shares)     (3,506,123)      
Forfeited (in shares)     (130,125)      
Outstanding at the end of the period (in shares) 4,123,216   4,123,216   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)     91.36      
Released (in dollars per share)     11.52      
Forfeited (in dollars per share)     54.17      
Outstanding at the end of the period (in dollars per share) $ 57.49   $ 57.49   $ 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)     (314,071)      
Forfeited (in shares)     (16,548)      
Outstanding at the end of the period (in shares) 2,386,986   2,386,986   2,531,561  
Exercisable at the end of the period (in shares) 1,431,015   1,431,015      
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)     15.91      
Forfeited (in dollars per share)     32.35      
Outstanding at the end of the period (in dollars per share) $ 23.84   23.84   $ 17.86  
Exercisable at the end of the period (in dollars per share) $ 14.35   $ 14.35      
Weighted Average Remaining Contractual Term            
Outstanding at the end of the period     6 years 7 months 6 days   6 years 3 months 18 days  
Exercisable at the end of the period     5 years 7 months 6 days      
Aggregate Intrinsic Value            
Outstanding at the end of the period $ 224,845   $ 224,845      
Exercisable at the end of the period 148,385   148,385      
Total intrinsic value of options exercised     22,600 $ 36,300    
Unrecognized compensation cost $ 198,900   $ 198,900      
Stock Plans | Maximum            
Aggregate Intrinsic Value            
Market price (in dollars per share)           $ 118.04