Related Party Transactions and Investments in Non-Consolidated Entities (Details 4) (USD $)	9 Months Ended			9 Months Ended		12 Months Ended	9 Months Ended	1 Months Ended			9 Months Ended				1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended			1 Months Ended	9 Months Ended			9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Sponsored REITs	Sep. 30, 2012 Acquisition Loan	Dec. 31, 2011 Acquisition Loan FSP Union Centre Corp loan	Sep. 30, 2012 Secured revolving lines of credit note	Mar. 31, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Feb. 28, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Jan. 31, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP High Land Place I Corp.	Mar. 02, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Feb. 02, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Jan. 02, 2011 Secured revolving lines of credit FSP High Land Place I Corp.	Mar. 31, 2011 Secured revolving lines of credit FSP Satellite Place Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP Satellite Place Corp.	Mar. 30, 2011 Secured revolving lines of credit FSP Satellite Place Corp.	Jan. 02, 2011 Secured revolving lines of credit FSP Satellite Place Corp.	Mar. 31, 2011 Secured revolving lines of credit FSP 1441 Main Street Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP 1441 Main Street Corp.	Mar. 30, 2011 Secured revolving lines of credit FSP 1441 Main Street Corp.	Jan. 02, 2011 Secured revolving lines of credit FSP 1441 Main Street Corp.	Mar. 31, 2011 Secured revolving lines of credit FSP 505 Waterford Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP 505 Waterford Corp.	Mar. 30, 2011 Secured revolving lines of credit FSP 505 Waterford Corp.	Jan. 02, 2011 Secured revolving lines of credit FSP 505 Waterford Corp.	Mar. 31, 2011 Secured revolving lines of credit FSP Phoenix Tower Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP Phoenix Tower Corp.	Mar. 30, 2011 Secured revolving lines of credit FSP Phoenix Tower Corp.	Jan. 02, 2011 Secured revolving lines of credit FSP Phoenix Tower Corp.	Sep. 30, 2012 Secured revolving lines of credit FSP Gallieria North Corp.	Sep. 30, 2012 Secured construction loan note	Mar. 31, 2009 Secured construction loan FSP 385 Interlocken Development Corp.	Mar. 30, 2011 Secured construction loan FSP 385 Interlocken Development Corp.	Sep. 30, 2012 Secured construction loan FSP 385 Interlocken Development Corp.	Jan. 02, 2011 Secured construction loan FSP 385 Interlocken Development Corp.	Jul. 31, 2012 Mortgage loan secured by property	Mar. 31, 2012 Mortgage loan secured by property	Dec. 31, 2011 Mortgage loan secured by property	Sep. 30, 2012 Mortgage loan secured by property FSP Energy Tower Corp.	Jul. 05, 2012 Mortgage loan secured by property FSP Energy Tower Corp.
Sponsored REITs
Number of sponsored REITs to which loans have been provided							6
Number of wholly-owned subsidiaries of the sponsored REITs to whom loan has been provided																																1
Commitment fee paid by borrower at loan origination																																	$ 210,000
Term of loan					2 years
Number of loans outstanding						1
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	133,800,000										5,500,000					5,500,000				10,800,000				7,000,000				15,000,000			15,000,000				42,000,000					33,000,000
Amount Drawn	108,236,000		140,516,000								1,125,000					5,500,000				8,000,000				2,350,000				15,000,000			5,720,000				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			30-day LIBOR	30-day LIBOR			30-day LIBOR	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%	3.00%	3.00%		4.40%	3.00%	3.00%		4.40%	3.00%	3.00%		4.40%	3.00%	3.00%		4.40%	3.00%	3.00%	5.00%			3.00%	4.40%	3.00%
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%			0.50%
Interest rate (as a percent)											4.63%					4.63%				4.63%				4.63%				4.63%			5.23%				4.63%					6.41%
Loan fee related to fixed mortgage component																																					300,630	300,000	762,000
Exit fees (as a percent)																																					0.98%
Payment of loan fee related to fixed mortgage component																																									300,630
Payment of loan fee related to fixed mortgage component																																					520,000
Amount drawn from revolving line of credit component																																						30,000,000
Repayment fee (as a percent)																																					0.49%			0.982%
Interest income and fees from the Sponsored REIT Loans	$ 8,299,000	$ 2,280,000