Fair Value of Financial Instruments (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Carrying Value	$ 213,898	$ 126,167
Senior, One Point Three Seven Five Percent Convertible Notes Due Two Thousand Twenty Three [Member] [Member]
Debt Instrument [Line Items]
Face Value	172,500	0
Carrying Value	135,447	0
Fair Value	173,450	0
1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Face Value		138,000
Carrying Value		126,167
Fair Value		$ 173,961
1.125% Convertible Senior Notes due 2018
Debt Instrument [Line Items]
Face Value	81,207
Carrying Value	78,451
Fair Value	$ 100,802