Credit Facility (Schedule of Future Principal Payments Due on Secured Credit Facility) (Details) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Debt Instrument [Line Items] | ||
| 2020 | $ 4,530 | |
| 2021 | 155,207 | |
| 2022 | 279,922 | |
| 2023 | 252,712 | |
| Total | $ 832,481 | |
| Secured Credit Facility [Member] | ||
| Debt Instrument [Line Items] | ||
| 2019 | $ 0 | |
| 2020 | 0 | |
| 2021 | 0 | |
| 2022 | 105,000 | |
| 2023 | 250,000 | |
| Total | $ 355,000 |