Segment Reporting (Narrative) (Details) |
3 Months Ended | 12 Months Ended | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Mar. 31, 2019
USD ($)
segment
|
Dec. 31, 2018
USD ($)
|
Sep. 30, 2018
USD ($)
|
Jun. 30, 2018
USD ($)
|
Mar. 31, 2018
USD ($)
|
Dec. 31, 2017
USD ($)
|
Sep. 30, 2017
USD ($)
|
Jun. 30, 2017
USD ($)
|
Mar. 31, 2017
USD ($)
|
Dec. 31, 2018
USD ($)
segment
|
Dec. 31, 2017
USD ($)
segment
|
Dec. 31, 2016
USD ($)
|
|
| Segment Reporting Information [Line Items] | ||||||||||||
| Number of reportable business segments | segment | 2 | 2 | 2 | |||||||||
| Rental revenue | $ 46,467,000 | $ 46,569,000 | $ 45,518,000 | $ 43,950,000 | $ 41,294,000 | $ 37,266,000 | $ 36,205,000 | $ 27,602,000 | $ 24,022,000 | $ 177,332,000 | $ 125,095,000 | $ 56,431,000 |
| Intersegment Elimination [Member] | ||||||||||||
| Segment Reporting Information [Line Items] | ||||||||||||
| Rental revenue | $ 0 | $ 0 | $ 0 | $ 0 | $ 0 | |||||||