Related Party Transactions and Investments in Non-Consolidated Entities (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2015 entity	Dec. 31, 2014 USD ($) entity	Sep. 30, 2014 entity
Related Party Transactions and Investments in Non-Consolidated Entities
Sponsored REITs (in entities)	9	10	14
Number of REITs in which the entity holds non-controlling preferred stock interest	2
Number of sponsored REITs having properties which were sold	1	4
Number of entities holding mortgage loans		2
FSP Galleria North Corp.
Investment in Sponsored REITs
Repayment of principal | $		$ 13,880,000
FSP Highland Place I Corp.
Investment in Sponsored REITs
Repayment of principal | $		$ 3,395,000