Notes Payable (Narrative) (Details) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
|---|---|---|---|
| Debt Instrument [Line Items] | |||
| Notes payable, principal amount outstanding | $ 467,481 | $ 467,786 | $ 468,135 |
| Notes Payable [Member] | |||
| Debt Instrument [Line Items] | |||
| Weighted Average Interest Rate | 4.40% |