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STOCK AWARD PLANS - Summary of Total Compensation Expense Recognized in Statements of Operations for Stock-Based Compensation (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 18,134 $ 12,318
Stock options    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense 5,002 3,002
Stock awards    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense 12,212 8,410
Company expense on ESPP discount    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 920 $ 906