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Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs - Property Sold (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
FSP 1441 Main Street Corp. |  |
Investment in Sponsored REITs |  |
Repayment of principal | $ 9,000,000 |
FSP 385 Interlocken Development, Corp. |  |
Investment in Sponsored REITs |  |
Repayment of principal |  | $ 37,500,000

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