Real Estate Partnerships (Narrative) (Details) (USD $)	5 Months Ended	12 Months Ended			0 Months Ended			3 Months Ended	12 Months Ended									90 Months Ended
	Dec. 31, 2010 sqft	Dec. 31, 2011 properties	Dec. 31, 2010	Dec. 31, 2009	Oct. 31, 2011 Newbury Street Partnership [Member] Building	May 26, 2011 Newbury Street Partnership [Member] sqft	May 26, 2010 Newbury Street Partnership [Member] sqft Building	Mar. 31, 2012 Newbury Street Partnership [Member]	Dec. 31, 2011 Newbury Street Partnership [Member]	Dec. 31, 2010 Newbury Street Partnership [Member]	Dec. 31, 2011 Federal / Lion Venture LP [Member] properties	Dec. 31, 2010 Federal / Lion Venture LP [Member]	Dec. 31, 2009 Federal / Lion Venture LP [Member]	Dec. 31, 2011 Loan Secured By Two Buildings [Member] Newbury Street Partnership [Member]	May 26, 2011 Loan Secured by One Building [Member] Newbury Street Partnership [Member]	Dec. 31, 2011 Taurus Investment Holdings [Member] Newbury Street Partnership [Member]	Dec. 31, 2011 Federal Realty Investment Trust [Member] Newbury Street Partnership [Member]	Dec. 31, 2011 Federal Realty Investment Trust [Member] Federal / Lion Venture LP [Member]	Dec. 31, 2011 Clarion [Member] Federal / Lion Venture LP [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest in partnership																15.00%	85.00%	30.00%	70.00%
Date of Acquisition of Newbury Partnership Property						May 26, 2011
Total number of properties in the portfolio		87									7
Sales of Real Estate					$ 44,000,000
Leasable area (square feet)	149,000					6,700	32,000
Business Acquisition Cost of Acquired Entity Cash Paid and Total Debt Assumed						6,200,000	17,500,000
Contributed towards acquisition of property						2,800,000	7,800,000											42,100,000
Equity Method Investment, Dividends or Distributions																		12,800,000
Our investment in real estate partnership	51,606,000	34,352,000	51,606,000							16,100,000	34,352,000	35,504,000
Secured loan						11,800,000								8,800,000	3,100,000
Debt Instrument, Maturity Date									May 25, 2012
Loan interest rate									LIBOR plus 400 basis points
Number of Real Estate Properties Sold					3
Extension Option Period									1 year
Debt Instrument, Basis Spread on Variable Rate						4.00%
Income (Loss) from Equity Method Investments		1,808,000	1,060,000	1,322,000					100,000	400,000	1,771,000	1,449,000	1,322,000
Number of Real Estate Properties Acquired							2
Payments for (Proceeds from) Real Estate Partnership Investment, Net		(34,617,000)	0	0	34,600,000
Repayment Of Secured Debt FRT					11,800,000
Discontinued operation - gain on sale of real estate		15,075,000	1,000,000	1,298,000				11,800,000
Deferred Gain on Sale of Property									$ 11,800,000