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Fair Value of Financial Instruments (Details 2) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Debt Instrument, Face Amount $ 253,707 $ 253,707
Convertible Notes Payable 219,295 213,898
Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount 172,500 172,500
Convertible Notes Payable 138,647 135,447
Convertible Debt, Fair Value Disclosures 164,996 173,450
1.125% Convertible Senior Notes due 2018    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount 81,207 81,207
1.125% Convertible Senior Notes due 2018    
Debt Instrument [Line Items]    
Debt Instrument, Face Amount 81,207 81,207
Convertible Notes Payable 80,648 78,451
Convertible Debt, Fair Value Disclosures $ 90,749 $ 100,802