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STOCK AWARD PLANS - Summary of Employee Stock Purchase Plan Activity (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares purchased by employees (shares) 86,588  
Aggregate cost to employees $ 6,403,183  
Expense recognized by the company 17,123,000 $ 18,134,000
Company expense on ESPP discount    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expense recognized by the company $ 1,129,973 $ 920,000