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EMPLOYEE BENEFIT PLANS - Summary of changes in non-vested restricted shares (Details) - Restricted Stock - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Shares      
Non-vested shares 1,200,900 1,254,200  
Granted 202,400 371,950  
Vested (344,250) (422,533)  
Forfeited (18,250) (2,717)  
Non-vested shares 1,040,800 1,200,900 1,254,200
Weighted Average Grant-Date Fair Value      
Non-vested shares $ 6.58 $ 6.80  
Granted 13.39 5.77 $ 7.08
Vested 6.92 6.52  
Forfeited 8.47 6.21  
Non-vested shares $ 7.76 $ 6.58 $ 6.80