LONG-TERM DEBT - Convertible Notes (Details)		3 Months Ended
	Mar. 11, 2015 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares
Long-term debt
Face amount of debt		$ 203,100,230
Unamortized debt discount of notes		2,613,923
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%
Debt instrument conversion ratio	0.0145136	0.0145439
Conversion price per share | $ / shares	$ 68.90	$ 68.68
Unamortized debt discount of notes	$ 6,100,000	$ 2,600,000
Debt discount, cash portion	2,600,000
Debt discount, equity component	$ 3,500,000