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STOCK-BASED COMPENSATION - Schedule of Weighted-Average Assumptions for Options and ESPP Subscriptions Granted Granted (Details) - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Option Awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Average risk-free interest rate 2.50% 2.60%
Expected dividend yield 0.00% 0.00%
Expected volatility (as a percent) 29.00% 33.00%
Expected term 5 years 2 months 12 days 4 years 8 months 12 days
Fair value, per option (in dollars per share) $ 54.03 $ 43.45
ESPP    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Average risk-free interest rate 2.40% 1.50%
Expected dividend yield 0.00% 0.00%
Expected volatility (as a percent) 33.00% 33.00%
Expected term 7 months 6 days 7 months 6 days
Fair value, per option (in dollars per share) $ 48.83 $ 35.07