Related Party Transactions and Investments in Non-Consolidated Entities - Sponsored REIT Loans outstanding (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) loan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Sponsored REITs
Number of Sponsored REIT loans which bear interest at a fixed rate | loan	2
Maximum amount of loan	$ 79,500,000
Amount Drawn	71,720,000	$ 81,780,000
Interest income and fees from the Sponsored REIT Loans	$ 4,687,000	$ 4,834,000	$ 5,230,000
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
Secured revolving lines of credit | FSP Satellite Place Corp.
Sponsored REITs
Maximum amount of loan	$ 5,500,000
Amount Drawn	$ 2,120,000
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)	5.78%
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)	6.38%
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
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