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STOCK AWARD PLANS - Summary of Employee Stock Purchase Plan Activity (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares purchased by employees (shares) 65,416  
Aggregate cost to employees $ 5,211  
Expense recognized by the company 18,134 $ 12,318
Company expense on ESPP discount    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expense recognized by the company $ 920 $ 906