Debt - Convertible Senior Note (Details) - Convertible debt - USD ($) |
Sep. 30, 2019 |
Dec. 31, 2018 |
Jul. 31, 2015 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Principal | $ 225,000,000 | $ 225,000,000 | $ 225,000,000 |
| Unamortized debt discount | (4,000,000) | (8,000,000) | |
| Unamortized financing costs | (1,000,000) | (2,000,000) | |
| Carrying value of convertible senior notes | 220,000,000 | 215,000,000 | |
| Carrying value of the equity component | 24,000,000 | $ 24,000,000 | |
| Equity component of convertible debt issuance costs | $ 1,000,000 |