Related Party Transactions and Investments in Non-Consolidated Entities - Investment in Sponsored REITs - Property Sold (Details) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2017	Dec. 31, 2016
FSP 385 Interlocken Development, Corp.
Investment in Sponsored REITs
Repayment of principal	$ 37,500,000	$ 37,500,000
FSP 1441 Main Street Corp.
Investment in Sponsored REITs
Repayment of principal	$ 9,000,000	$ 9,000,000

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