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EMPLOYEE STOCK COMPENSATION - Summary of Stock-based Compensation Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense $ 12,134 $ 11,947 $ 9,328
Related tax benefits 5,078 4,963 3,922
Stock option awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense 360 531 528
Restricted stock awards and restricted stock units      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense 11,732 11,398 8,598
Stock appreciation rights      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Total stock-based compensation expense $ 42 $ 18 $ 202