Convertible Senior Notes - Components of Total Interest Expense, Net Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Contractual coupon interest expense	$ 2,615	$ 3,288	$ 3,300
Amortization of debt issuance costs	567	696	662
Accretion of debt discount	4,890	5,710	5,250
Total	$ 8,072	$ 9,694	$ 9,212
Effective interest rate	8.41%	8.65%	8.66%