Debt - Convertible Senior Note (Details) - Convertible debt [Member] - USD ($) $ in Thousands |
Mar. 31, 2018 |
Dec. 31, 2017 |
Jul. 31, 2015 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Principal of convertible senior notes | $ 225,000 | $ 225,000 | $ 225,000 |
| Unamortized debt discount | (12,243) | (13,470) | |
| Unamortized financing costs | (2,523) | (2,794) | |
| Carrying value of convertible senior notes | 210,234 | 208,736 | |
| Carrying value of the equity component | 23,743 | $ 23,743 | |
| Equity component of convertible debt issuance costs | $ 700 |