Debt (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified					0 Months Ended			9 Months Ended		9 Months Ended				0 Months Ended			0 Months Ended					0 Months Ended							0 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member]	Mar. 25, 2013 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Sep. 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]	Sep. 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]	Sep. 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]	Sep. 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]	Sep. 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]	Sep. 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]	Sep. 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]	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member] Subsequent Event [Member] Tranches	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Skyline Properties Senior Position [Member] Subsequent Event [Member]	Oct. 31, 2013 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Skyline Properties Junior Position [Member] Subsequent Event [Member]	Sep. 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]	Sep. 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	Sep. 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			2.39%					1.58%		2.36%		4.91%			3.61%			3.56%		5.74%		5.74%	2.965%		5.43%		4.25%			1.33%
Face amount of mortgage loan																						678,000	350,000	328,000
Number Of Loan Tranches																						2
Maturity, Range, Start																						February 2017
Maturity, Range, End																						February 2022
Length Of Extension Available																						1 year							6 months
Floating Rate Loan On Property					282,312
Spread Over LIBOR								1.53%		1.36%
LIBOR Floor										1.00%
Mortgages payable (in US Dollars)	$ 8,566,582	$ 8,644,200	$ 1,269,115	$ 1,873,199		$ 0	$ 282,312	$ 64,000	$ 64,000	$ 60,000	$ 60,000	$ 7,297,467	$ 6,771,001		$ 390,000	$ 0		$ 300,000	$ 0	$ 736,259	$ 704,957				$ 195,546	$ 195,546	$ 563,692	$ 573,180
Debt Instrument, Description of Variable Rate Basis								Freddie Mac		LIBOR
Number Of Extensions Available																													2
Facility fee percent																														0.20%	0.30%