Related Party Transactions and Investments in Non-Consolidated Entities (Details 4) - USD ($)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Sponsored REITs
Maximum amount of loan	$ 111,300,000
Amount Drawn	93,641,000		$ 93,641,000
Interest income and fees from the Sponsored REIT Loans	$ 3,833,000	$ 4,032,000
Sponsored REITs
Sponsored REITs
Term of sponsored REIT loan secured by mortgage, minimum	1 year
Term of sponsored REIT loan secured by mortgage, maximum	3 years
Extended term of sponsored REIT loan secured by mortgage, minimum	1 year
Secured revolving lines of credit | FSP Satellite Place Corp.
Sponsored REITs
Maximum amount of loan	$ 5,500,000
Amount Drawn	$ 5,500,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	4.60%
Secured revolving lines of credit | FSP 1441 Main Street Corp.
Sponsored REITs
Maximum amount of loan	$ 10,800,000
Amount Drawn	$ 9,000,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	4.60%
Secured revolving lines of credit | FSP Energy Tower I Corp.
Sponsored REITs
Maximum amount of loan	$ 20,000,000
Amount Drawn	$ 8,600,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	5.00%
Draw Fee (as a percent)	0.50%
Interest rate (as a percent)	5.20%
Secured construction loan | FSP 385 Interlocken Development Corp.
Sponsored REITs
Maximum amount of loan	$ 42,000,000
Amount Drawn	$ 37,541,000
Sponsored REIT loans, base rate	30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)	4.40%
Interest rate (as a percent)	4.60%
Mortgage loan secured by property | FSP Energy Tower I Corp.
Sponsored REITs
Maximum amount of loan	$ 33,000,000
Amount Drawn	$ 33,000,000
Fixed rate of interest (as a percent)	6.41%
Interest rate (as a percent)	6.41%