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LONG-TERM DEBT - Convertible Notes (Details)
3 Months Ended
Mar. 11, 2015
USD ($)
$ / shares
Mar. 31, 2019
USD ($)
$ / shares
Dec. 31, 2018
Long-term debt      
Face amount of debt   $ 209,232,846  
Unamortized debt discount of notes   1,320,009  
4.50% Convertible Senior Notes due 2020      
Long-term debt      
Face amount of debt $ 75,000,000 $ 75,000,000  
Stated interest rate (as a percent)   4.50% 4.50%
Debt instrument conversion ratio 0.0145136 0.0146162  
Conversion price per share | $ / shares $ 68.90 $ 68.42  
Sinking fund provided   $ 0  
Unamortized debt discount of notes $ 6,100,000 $ 1,300,000  
Debt discount, cash portion 2,600,000    
Debt discount, equity component $ 3,500,000