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STOCK-BASED COMPENSATION - Schedule of Weighted-Average Assumptions for Options and ESPP Subscriptions Granted Granted (Details) - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Option Awards        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Average risk-free interest rate 2.90% 1.80% 2.90% 1.80%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Expected volatility (as a percent) 29.00% 33.00% 29.10% 33.00%
Expected term 5 years 4 years 6 months 5 years 4 years 6 months
Fair value, per option (in dollars per share) $ 42.12 $ 33.64 $ 42.14 $ 33.54
ESPP        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Average risk-free interest rate 1.00% 0.50% 0.90% 0.40%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%
Expected volatility (as a percent) 32.80% 32.80% 32.60% 32.90%
Expected term 7 months 6 days 7 months 6 days 7 months 6 days 7 months 6 days
Fair value, per option (in dollars per share) $ 36.94 $ 29.89 $ 35.49 $ 25.19