Debt - Additional Information - Convertible Senior Notes (Details) - Convertible Senior Notes |
12 Months Ended | ||
|---|---|---|---|
|
May 20, 2014
USD ($)
$ / shares
|
Dec. 31, 2017
USD ($)
|
Dec. 31, 2016
USD ($)
|
|
| Debt Instrument [Line Items] | |||
| Conversion rate | 0.0763636 | 0.0773144 | |
| Conversion price (in dollars per share) | $ / shares | $ 13.10 | ||
| Premium above public offering price | 22.50% | ||
| Cash dividend per share (in dollars per share) | $ / shares | $ 0.16625 | ||
| Value of the embedded conversion premium, included in net debt (discount) | $ 56,700,000 | $ 23,700,000 | $ 33,500,000 |
| Unamortized deferred financing costs | 19,600,000 | ||
| Deferred financing costs | $ 8,000,000 | $ 11,400,000 | |
| Convertible Senior Notes Due 2019 | |||
| Debt Instrument [Line Items] | |||
| Aggregate principal amount of debt | $ 402,500,000.0 | ||
| Weighted average stated interest rate | 2.875% | ||
| Convertible Senior Notes Due 2021 | |||
| Debt Instrument [Line Items] | |||
| Aggregate principal amount of debt | $ 345,000,000.0 | ||
| Weighted average stated interest rate | 3.75% |