Related Party Transactions and Investments in Non-Consolidated Entities (Details) (USD $)	3 Months Ended				6 Months Ended			6 Months Ended		1 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Mar. 31, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Sponsored REITs (no non-controlling preferred stock investment)	Jun. 30, 2010 Sponsored REITs (no non-controlling preferred stock investment)	Sep. 30, 2006 FSP Phoenix Tower Corp	Jun. 30, 2011 FSP Phoenix Tower Corp	Jun. 30, 2010 FSP Phoenix Tower Corp	Dec. 31, 2007 East Wacker	Jun. 30, 2011 East Wacker	Jun. 30, 2010 East Wacker	May 31, 2009 Grand Boulevard	Jun. 30, 2011 Grand Boulevard	Jun. 30, 2010 Grand Boulevard
Related Party Transactions and Investments in Non-Consolidated Entities
Sponsored REITs	16		14		16	14	15
Fully syndicated	15				15
Not fully syndicated	1				1
Number of REITs in which the entity holds non-controlling preferred stock interest	3				3
Sponsored REIT
Equity in earnings	$ 1,166,000		$ 380,000		$ 2,134,000	$ 633,000		$ 948,000	$ 162,000		$ (11,000)	$ (9,000)		$ 1,186,000	$ 367,000		$ 11,000	$ 113,000
Preferred shares purchased (in shares)										48			965.75			175.5
Percentage of outstanding preferred shares purchased (as a percent)										4.60%			43.70%			27.00%
Net cost of preferred shares purchased										4,116,000			82,813,000			15,049,000
Offering price of preferred shares purchased										4,800,000			96,575,000			17,550,000
Commissions excluded										384,000			7,726,000			1,404,000
Loan fees excluded													5,553,000			1,009,000
Acquisition fees excluded										300,000			483,000			88,000
Distributions from non-consolidated REITs	$ 1,215,000	$ 1,767,000	$ 1,324,000	$ 1,407,000	$ 2,982,000	$ 2,731,000