Debt - Convertible Debt (Details) - Convertible Debt - 0.5% Convertible Senior Notes Due 2033 - USD ($) $ in Thousands |
Jan. 31, 2016 |
Dec. 31, 2013 |
|---|---|---|
| Debt Instrument | ||
| Principal | $ 258,750 | $ 258,750 |
| Unamortized debt discount | (30,189) | |
| Net carrying amount of the liability component | 228,561 | |
| Carrying amount of the equity component | $ 49,648 |