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DEBT (Details) - USD ($)
Mar. 13, 2017
Jun. 30, 2018
Dec. 31, 2017
Convertible Senior Notes Due 2022      
Debt Instrument [Line Items]      
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock   $ 1,000  
Unsecured Debt | Convertible Senior Notes Due 2022      
Debt Instrument [Line Items]      
Convertible senior notes, gross   345,000,000 $ 345,000,000
Deferred financing costs   (6,675,000) (7,482,000)
Discount on debt   (55,067,000) (61,345,000)
Total debt, net of debt discount and deferred financing costs   283,258,000 276,173,000
Unsecured Debt | Convertible Senior Notes Due 2019      
Debt Instrument [Line Items]      
Convertible senior notes, gross   338,000 338,000
Deferred financing costs   (1,000) (2,000)
Discount on debt   (7,000) (12,000)
Total debt, net of debt discount and deferred financing costs   330,000 $ 324,000
Debt Conversion Terms Business Day Immediately Preceding October 1, 2021 [Member] | Convertible Senior Notes Due 2022      
Debt Instrument [Line Items]      
Debt Instrument, Conversion Obligation Common Stock Closing Sales Price Minimum as Percentage of Conversion Price 130.00%    
Debt Instrument, Principal Amount Denominator for Conversion into Common Stock   $ 1,000  
Debt Instrument, Conversion Obligation Common Stock Closing Sales Price Minimum, Number of Trading Days 20 days    
Debt Instrument, Conversion Obligation Number of Consecutive Trading Days 30 days