Convertible Senior Notes Schedule of Interest Expense Related to Convertible Senior Notes (Details) - Convertible Senior Notes - Convertible Senior Notes 0.25% Due May 15, 2018 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Debt Instrument
0.25% coupon	$ 188	$ 187
Amortization of debt issuance costs	323	305
Amortization of debt discount	3,209	3,035
Interest expense, convertible notes	$ 3,720	$ 3,527