Senior Notes Carrying value of the Senior Notes (Details) - USD ($) |
Sep. 30, 2018 |
Dec. 31, 2017 |
|---|---|---|
| Debt Instrument [Line Items] | ||
| Debt issuance costs, net of accumulated amortization of $44 | $ 0 | $ (967,000) |
| Accumulated Amortization, Debt Issuance Costs | 4,518,000 | $ 4,442,000 |
| Senior Notes [Member] | ||
| Debt Instrument [Line Items] | ||
| Principal amount | 325,000,000 | |
| Debt discount, net of accumulated amortization of $154 | (4,721,000) | |
| Debt issuance costs, net of accumulated amortization of $44 | (1,323,000) | |
| Carrying value of the Senior Notes | 318,956,000 | |
| Accumulated amortization, debt discount | 38,000 | |
| Accumulated Amortization, Debt Issuance Costs | $ 11,000 |