Long-Term Debt (Details) - USD ($) |
1 Months Ended | 3 Months Ended | ||
|---|---|---|---|---|
Aug. 31, 2017 |
Mar. 31, 2018 |
Dec. 31, 2017 |
Sep. 30, 2012 |
|
| Debt Instrument [Line Items] | ||||
| Long-term debt, fair value | $ 289,831,000 | $ 290,824,000 | ||
| US Treasury Note | ||||
| Debt Instrument [Line Items] | ||||
| Term of note used (years) | 10 years | |||
| Effective percentage (percent) | 2.741% | |||
| Revolving Credit Facility | ||||
| Debt Instrument [Line Items] | ||||
| Credit facility capacity | $ 50,000,000 | |||
| Term of revolving credit facility (years) | 3 years | |||
| Senior Notes Due 2022 | Senior Notes | ||||
| Debt Instrument [Line Items] | ||||
| Principal | $ 275,000,000 | $ 275,000,000 | $ 275,000,000 | |
| Coupon rate (percent) | 5.00% | |||
| Capitalized debt issuance costs | $ 2,200,000 | |||
| Long-term debt, fair value | $ 286,300,000 | |||
| 10-year U.S. Treasury note | Senior Notes Due 2022 | Senior Notes | ||||
| Debt Instrument [Line Items] | ||||
| Basis points | 1.24% |