Related Party Transactions and Investments in Non-Consolidated Entities - Sponsored REIT Loans outstanding (Details)	3 Months Ended
	Mar. 31, 2017 USD ($) loan	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Sponsored REITs
Number of Sponsored REIT loans which bear interest at a fixed rate | loan	2
Maximum amount of loan	$ 90,300,000
Amount Drawn	$ 81,515,000		$ 81,780,000
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
Interest income and fees from the Sponsored REIT Loans	$ 1,214,000	$ 1,279,000
Secured revolving lines of credit | FSP Satellite Place Corp.
Sponsored REITs
Maximum amount of loan	5,500,000
Amount Drawn	$ 2,915,000
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)	5.21%
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)	5.21%
Secured revolving lines of credit | FSP Energy Tower I Corp.
Sponsored REITs
Maximum amount of loan	$ 20,000,000
Amount Drawn	$ 15,600,000
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)	5.81%
Mortgage loan secured by property | FSP Monument Circle LLC
Sponsored REITs
Maximum amount of loan	$ 21,000,000
Amount Drawn	$ 21,000,000
Fixed rate of interest (as a percent)	4.90%
Interest rate (as a percent)	4.90%
Origination fee	$ 164,000
Exit fee	38,000
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%
Extension fee	$ 108,900