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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