Real Estate (Significant Acquisitions and Disposition) (Details) (USD $)	5 Months Ended	12 Months Ended			0 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended		0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended		0 Months Ended									0 Months Ended
	Dec. 31, 2010 sqft	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Jul. 12, 2011 Feasterville Shopping Center 7/12/11 [Member]	Sep. 30, 2011 Feasterville Shopping Center 7/12/11 [Member]	Dec. 31, 2010 Property In Northern Virginia [Member]	Dec. 31, 2010 Fifth Avenue Buildings [Member] Building	Dec. 31, 2010 Fifth Avenue Building One [Member] Building	Jan. 19, 2011 Tower Shops January 19 2011 [Member] sqft acre	Dec. 31, 2011 Tower Shops January 19 2011 [Member]	Dec. 27, 2011 Montrose Crossing [Member] sqft	Dec. 31, 2011 Montrose Crossing [Member]	Dec. 30, 2011 Plaza El Segundo [Member] sqft	Dec. 31, 2011 Plaza El Segundo [Member]	Dec. 30, 2011 Land Parcel Adjacent To Plaza El Segundo [Member] acre	Aug. 16, 2010 HuntingtonSquare [Member] sqft		Nov. 10, 2010 Former Mervyn’s Parcel (Escondido Promenade) [Member] sqft		Nov. 22, 2010 Pentagon Row [Member]		Dec. 27, 2010 Bethesda Row [Member] acre		Dec. 27, 2011 New Mortgage Debt [Member] Montrose Crossing [Member]	Nov. 22, 2010 Termination Of Lease [Member] Pentagon Row [Member]
Real Estate Investments [Line Items]
Business Acquisition Cost of Acquired Entity Cash Paid and Share of Debt Assumed	$ 46,700,000		$ 46,700,000							$ 66,100,000		$ 127,200,000				$ 15,900,000	$ 17,600,000	[1]	$ 11,200,000	[2]	$ 8,500,000	[3]	$ 9,400,000	[4]
Mortgage loan face amount										41,000,000				175,000,000
Mortgage loan fair value										42,900,000															80,000,000
Leasable area (square feet)	149,000									368,000		357,000		381,000			74,000		75,000
Business Acquisition, Cost of Acquired Entity, Cash Paid														8,500,000
Acres of real estate property										67						8.1							2.1	[4]
Business Acquisition, Purchase Price Allocation, Amortizable Intangible Assets										1,200,000							9,200,000	[1]
Business Acquisition Purchase Price Allocation Amortizable Intangible Below Market Lease Liabilities										4,400,000							1,700,000	[1]
Acquisition costs										400,000	200,000		2,400,000		1,000,000		300,000
Proceeds from sale of buildings					20,000,000
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax		15,075,000	1,000,000	1,298,000		14,800,000
Business Acquisition, Percentage of Voting Interests Acquired												89.90%		48.20%
Cash Flow Of Controlled Entity, Percent												89.90%
Business Acquisition Cost of Acquired Entity Cash Paid and Total Debt Assumed												141,500,000
Business Acquisition Purchase Price														192,700,000
Other Significant Noncash Transaction, Value of Consideration Received																										8,800,000
Excess Of Relieved Liability Over Purchase Price																										300,000
Gain (Loss) on Sale of Properties, Net of Applicable Income Taxes		0	410,000	0			400,000
Number Of Buildings Under Plan Of Sale								2	1
Discontinued Operation, Provision for Loss (Gain) on Disposal, Net of Tax									$ 400,000

[1] We acquired the leasehold interest in this property. Approximately $9.2 million of net assets acquired were allocated to other assets for “above market leases” and a “below market ground lease” for which we are the lessee. Approximately $1.7 million of net assets acquired were allocated to liabilities for “below market leases”. We incurred approximately $0.3 million of acquisition costs which are included in “general and administrative expenses”.

[2] This property is adjacent to and operated as part of Escondido Promenade which is owned through a partnership in which we own the controlling interest.

[3] We and a subsidiary of Post Properties, Inc. (“Post”) purchased the fee interest in the land under Pentagon Row. The land was purchased as a result of a favorable outcome to litigation. In September 2008, we and Post sued Vornado Realty Trust and related entities (“Vornado”) for breach of contract in the Circuit Court of Arlington County, Virginia. The breach of contract was a result of Vornado’s acquiring in transactions in 2005 and 2007 the fee interest in the land under our Pentagon Row project without first giving us and Post the opportunity to purchase the fee interest in that land as required by the right of first offer (“ROFO”) provisions included in the documentation relating to the Pentagon Row project. On April 30, 2010, the judge in this case issued a ruling that Vornado failed to comply with the ROFO and as a result, breached the contract, and ordered Vornado to sell to us and Post, collectively, the land under Pentagon Row. Vornado appealed the ruling, however, the appeal was denied in November 2010. As part of the acquisition of the land and termination of the respective ground lease, we were relieved of our deferred ground rent liability for approximately $8.8 million. The liability was offset against the purchase price with the excess of the liability over the purchase price of $0.3 million included in the statement of operations as an adjustment to rental expense.

[4] We acquired the fee interest in approximately 2.1 acres of land under Bethesda Row. Prior to the transaction, the land parcel was owned pursuant to a ground lease and encumbered by a capital lease obligation which was terminated as part of the transaction.