Fair Value of Financial Instruments (Details 2) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 253,707	$ 253,707
Convertible Notes Payable	216,578	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	137,037	135,447
Convertible Debt, Fair Value Disclosures	170,404	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	79,541	78,451
Convertible Debt, Fair Value Disclosures	$ 87,866	$ 100,802