Long-Term Debt - Long-term Debt Less Unamortized Debt Issuance Costs (Details) - USD ($) |
Mar. 31, 2018 |
Dec. 31, 2017 |
Sep. 30, 2012 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Long-term debt less unamortized debt issuance costs | $ 273,865,000 | $ 273,809,000 | |
| Senior Notes | Senior Notes Due 2022 | |||
| Debt Instrument [Line Items] | |||
| Principal | 275,000,000 | 275,000,000 | $ 275,000,000 |
| Unamortized debt issuance costs | 1,135,000 | 1,191,000 | |
| Long-term debt less unamortized debt issuance costs | $ 273,865,000 | $ 273,809,000 |