Summary of Significant Accounting Policies - Reconciliation of Beginning and Ending Balances of Assets Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Level 3) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loss on extinguishment of convertible senior notes	$ 453	$ 611	$ (98)
Change in fair value	(2,948)	308
3.25% Convertible senior notes due in 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Issuance of 3.25% convertible senior notes	8,000	21,550
3.25% Convertible senior notes due in 2020 | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Convertible senior notes, beginning balance	22,469	0
Convertible senior notes, ending balance	$ 27,974	$ 22,469	$ 0