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Real Estate Investments - Purchase Price Allocation (Parenthetical) (Details) - USD ($) $ in Thousands | Jan. 05, 2018 | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Business Acquisition [Line Items] |  |  |
Outstanding principal balance on mortgage debt assumed |  | $ 322,588 | $ 317,216
Four Tower Bridge [Member] | Intangible Assets Acquired [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Weighted average amortization period | 4 years 1 month 6 days |  |
Four Tower Bridge [Member] | Below Market Lease [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Weighted average amortization period | 4 years 9 months 18 days |  |
Four Tower Bridge [Member] | Mortgage Debt Assumed - at Fair Value [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Outstanding principal balance on mortgage debt assumed | $ 9,700 |  |

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