Convertible Senior Notes - Components of Total Interest Expense, Net Recognized (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Contractual coupon interest expense	$ 981	$ 2,615	$ 3,288
Amortization of debt issuance costs	220	567	696
Accretion of debt discount	1,931	4,890	5,710
Total	$ 3,132	$ 8,072	$ 9,694
Effective interest rate	8.41%	8.41%	8.65%