Intangible Lease Liabilities, Net (Narrative) (Details) - USD ($) $ in Thousands |
3 Months Ended | 12 Months Ended | |||
|---|---|---|---|---|---|
Mar. 31, 2019 |
Mar. 31, 2018 |
Dec. 31, 2018 |
Dec. 31, 2017 |
Dec. 31, 2016 |
|
| Intangible Lease Liabilities, Net [Abstract] | |||||
| Amortization of below-market leases | $ 1,232 | $ 1,221 | $ 4,832 | $ 2,126 | $ 536 |