Debt (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified							6 Months Ended		6 Months Ended				0 Months Ended			0 Months Ended			6 Months Ended						0 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member]	Jun. 30, 2013 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Jun. 30, 2013 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Jun. 30, 2013 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member]	Feb. 20, 2013 Notes And Mortgages Payable Fixed Rate [Member] New York Office 666 Fifth Avenue [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member] New York Office 666 Fifth Avenue [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York Office 666 Fifth Avenue [Member]	Mar. 25, 2013 Notes And Mortgages Payable Fixed Rate [Member] Retail Bergen Town Center [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member] Retail Bergen Town Center [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Bergen Town Center [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Jun. 30, 2013 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Mar. 28, 2013 Unsecured Revolving Credit Facilities [Member] Extensions	Jun. 30, 2013 Unsecured Revolving Credit Facilities [Member]	Dec. 31, 2012 Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Notes And Loans Payable Refinanced Amount (in US Dollars)													$ 390,000			$ 300,000
Debt Term													10 years			10 years
Interest Rate, End of Period			2.42%				1.55%		2.36%		4.91%			3.61%			3.56%		5.74%		5.43%		4.25%			1.32%
Vacancy Rate																			45.20%
Interest Payments Deferred (in US Dollars)																			47,559
Spread Over LIBOR							1.53%		1.36%
LIBOR Floor									1.00%
Mortgages payable (in US Dollars)	$ 8,582,573	$ 8,663,326	$ 1,269,760	$ 1,892,325	$ 0	$ 282,312	$ 64,000	$ 64,000	$ 60,000	$ 60,000	$ 7,312,813	$ 6,771,001		$ 390,000	$ 0		$ 300,000	$ 0	$ 725,559	$ 704,957	$ 195,546	$ 195,546	$ 566,886	$ 573,180
Debt Instrument, Description of Variable Rate Basis							Freddie Mac		LIBOR
Number Of Extensions Available																									2
Length Of Extension Available																									6 months
Facility fee percent																										0.20%	0.30%