Fair Value (Narrative) (Details) - USD ($)
$ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
Dec. 31, 2017 |
| Fair Value [Line Items] |
|
|
|
| Notes payable, principal amount outstanding |
$ 467,481
|
$ 467,786
|
$ 468,135
|
| Secured credit facility, principal amount outstanding |
365,000
|
355,000
|
220,000
|
| Fixed Rate [Member] |
|
|
|
| Fair Value [Line Items] |
|
|
|
| Notes payable, principal amount outstanding |
220,276
|
220,351
|
220,436
|
| Variable Rate, Subject To Interest Rate Swap [Member] |
|
|
|
| Fair Value [Line Items] |
|
|
|
| Notes payable, principal amount outstanding |
247,205
|
247,435
|
247,699
|
| Secured credit facility, principal amount outstanding |
100,000
|
100,000
|
100,000
|
| Variable Rate [Member] |
|
|
|
| Fair Value [Line Items] |
|
|
|
| Secured credit facility, principal amount outstanding |
265,000
|
255,000
|
120,000
|
| Estimate of Fair Value Measurement [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Rate [Member] |
|
|
|
| Fair Value [Line Items] |
|
|
|
| Notes payable, fair value disclosure |
216,542
|
214,282
|
211,011
|
| Estimate of Fair Value Measurement [Member] | Significant Other Observable Inputs (Level 2) [Member] | Variable Rate, Subject To Interest Rate Swap [Member] |
|
|
|
| Fair Value [Line Items] |
|
|
|
| Notes payable, fair value disclosure |
243,258
|
241,739
|
243,812
|
| Secured credit facility, fair value disclosure |
$ 96,554
|
$ 96,146
|
$ 98,593
|