Acquired Intangible Assets, 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 |
| Acquired Finite-Lived Intangible Assets [Line Items] |
|
|
|
|
|
| Acquired intangible asset, weighted average remaining life |
9 years 8 months 12 days
|
|
10 years 7 months 6 days
|
11 years 2 months 12 days
|
|
| Depreciation and amortization |
$ 18,246
|
$ 13,717
|
$ 58,258
|
$ 41,133
|
$ 19,211
|
| In-place leases and Above-market leases [Member] |
|
|
|
|
|
| Acquired Finite-Lived Intangible Assets [Line Items] |
|
|
|
|
|
| Depreciation and amortization |
$ 7,795
|
|
|
|
|
| In-place leases, Above-market leases and Ground lease assets [Member] |
|
|
|
|
|
| Acquired Finite-Lived Intangible Assets [Line Items] |
|
|
|
|
|
| Depreciation and amortization |
|
$ 4,694
|
$ 19,919
|
$ 14,167
|
$ 5,987
|
| In-place leases [Member] |
|
|
|
|
|
| Acquired Finite-Lived Intangible Assets [Line Items] |
|
|
|
|
|
| Acquired intangible asset, weighted average remaining life |
9 years 9 months 18 days
|
|
10 years 1 month 6 days
|
11 years
|
|
| Impairment of intangible assets |
$ 2,658
|
|
|
|
|