Related Party Transactions and Investments in Non-Consolidated Entities (Details 4) (USD $)	3 Months Ended			3 Months Ended							3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Sponsored REITs	Mar. 31, 2013 Secured revolving lines of credit FSP High Land Place I Corp.	Mar. 31, 2013 Secured revolving lines of credit FSP Satellite Place Corp.	Mar. 31, 2013 Secured revolving lines of credit FSP 1441 Main Street Corp.	Mar. 31, 2013 Secured revolving lines of credit FSP 505 Waterford Corp.	Mar. 31, 2013 Secured revolving lines of credit FSP Galleria North Corp.	Dec. 31, 2012 Secured revolving lines of credit FSP Galleria North Corp.	Mar. 31, 2013 Secured construction loan FSP 385 Interlocken Development Corp.	Mar. 31, 2013 Mortgage loan secured by property FSP Energy Tower Corp.	Jul. 05, 2012 Mortgage loan secured by property FSP Energy Tower Corp.
Sponsored REITs
Term of sponsored REIT loan secured by mortgage, minimum				2 years
Term of sponsored REIT loan secured by mortgage, maximum				3 years
Maximum amount of loan	$ 118,800,000				$ 5,500,000	$ 5,500,000	$ 10,800,000	$ 7,000,000	$ 15,000,000		$ 42,000,000	$ 33,000,000
Amount Drawn	96,896,000		93,896,000		1,125,000	5,500,000	9,000,000	2,350,000	8,380,000		37,541,000	33,000,000
Sponsored REIT loans, base rate					30-day LIBOR	30-day LIBOR	30-day LIBOR	30-day LIBOR	30-day LIBOR		30-day LIBOR
Sponsored REIT loans, base rate margin (as a percent)					4.40%	4.40%	4.40%	4.40%	5.00%		4.40%
Fixed rate of interest (as a percent)												6.41%
Draw Fee (as a percent)					0.50%	0.50%	0.50%	0.50%	0.50%	0.50%
Interest rate (as a percent)					4.60%	4.60%	4.60%	4.60%	5.20%		4.60%	6.41%
Exit fees (as a percent)												0.982%
Interest income and fees from the Sponsored REIT Loans	1,352,000	2,327,000
Payment of loan fee related to fixed mortgage component													$ 300,630