Related Party Transactions and Investments in Non-Consolidated Entities (Details 3) (USD $)			6 Months Ended		12 Months Ended	6 Months Ended	1 Months Ended		4 Months Ended	6 Months Ended				3 Months Ended	6 Months Ended		4 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended		3 Months Ended	6 Months Ended		6 Months Ended	1 Months Ended	3 Months Ended	6 Months Ended		6 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Acquisition Loan	Jun. 30, 2011 Acquisition Loan FSP Union Centre Corp	Dec. 31, 2010 Acquisition Loan FSP Monument Circle Corp.	Jun. 30, 2011 Sponsored REIT Revolver	Feb. 28, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Jan. 31, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Mar. 02, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Feb. 02, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Jan. 02, 2011 Sponsored REIT Revolver FSP High Land Place I Corp.	Mar. 31, 2011 Sponsored REIT Revolver FSP Satellite Place Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP Satellite Place Corp.	Jan. 02, 2011 Sponsored REIT Revolver FSP Satellite Place Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP 1441 Main Street Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP 1441 Main Street Corp.	Jan. 02, 2011 Sponsored REIT Revolver FSP 1441 Main Street Corp.	Mar. 31, 2011 Sponsored REIT Revolver FSP 505 Waterford Corp.	Jun. 30, 2011 Sponsored REIT Revolver FSP 505 Waterford Corp.	Jan. 02, 2011 Sponsored REIT Revolver FSP 505 Waterford Corp.	Mar. 31, 2011 Sponsored REIT Revolver FSP Phoenix Tower Corp	Jun. 30, 2011 Sponsored REIT Revolver FSP Phoenix Tower Corp	Jan. 02, 2011 Sponsored REIT Revolver FSP Phoenix Tower Corp	Jun. 30, 2011 Construction loan	Mar. 31, 2009 Construction loan FSP 385 Interlocken Development Corp.	Mar. 30, 2011 Construction loan FSP 385 Interlocken Development Corp.	Jun. 30, 2011 Construction loan FSP 385 Interlocken Development Corp.	Jan. 02, 2011 Construction loan FSP 385 Interlocken Development Corp.	Jun. 30, 2011 Sponsored REITs (no non-controlling preferred stock investment)	Jun. 30, 2010 Sponsored REITs (no non-controlling preferred stock investment)
Sponsored REITs
Term of loan (in years)			2
Number of loans outstanding				1	1
Number of sponsored REITs to which loans have been provided						5
Number of wholly-owned subsidiaries of the sponsored REITs to whom loan has been provided																										1
Term of sponsored REIT loan secured by mortgage, minimum (in years)																															2
Term of sponsored REIT loan secured by mortgage, maximum (in years)																															3
Maximum Amount of Loan									$ 5,500,000	$ 5,500,000					$ 5,500,000		$ 10,800,000	$ 10,800,000			$ 7,000,000			$ 15,000,000					$ 42,000,000		$ 85,800,000
Amount Drawn	61,916,000	57,684,000							1,125,000	1,125,000					5,500,000		7,300,000	7,300,000			1,150,000			9,300,000					37,541,000		61,916,000
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%	4.40%	3.00%	3.00%		4.40%	3.00%	4.40%	4.40%	3.00%		4.40%	3.00%		4.40%	3.00%				4.40%	3.00%
Draw Fee (as a percent)								0.50%	0.50%	0.50%					0.50%		0.50%	0.50%			0.50%			0.50%							0.50%
Interest rate (as a percent)									4.59%	4.59%					4.59%		4.59%	4.59%			4.59%			4.59%					4.59%
Commitment fee paid by borrower at loan origination																											210,000
Interest income and fees from the Acquisition Loan and Sponsored REIT Loans																															$ 1,494,000	$ 694,000