Debt (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended			0 Months Ended		3 Months Ended			0 Months Ended					0 Months Ended	9 Months Ended	1 Months Ended	9 Months Ended			1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member]	Jan. 09, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 350 Park Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 350 Park Avenue [Member]	Jun. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Mar. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Jul. 26, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2101 L Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2101 L Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member]	Mar. 05, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member] Extensions	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member]	Aug. 17, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue - Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue - Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Apr. 30, 2012 Exchangeable And Convertible Senior Debentures [Member]
Interest Rate, End of Period			5.40%		3.75%			5.74%		3.97%	5.43%	4.23%	2.50%		2.73%		2.47%	1.63%	2.36%
Notes And Loans Payable Refinanced Amount				$ 300,000					$ 150,000					$ 325,000		$ 98,000
Debt Amortization Period				30 years					30 years
Debt Term				5 years					12 years					3 years		7 years
Number Of Extensions Available														2
Length Of Extension Available In Years														1 year
Repayments of borrowings	2,070,295	2,666,610		132,000
Vacancy Rate							26.00%
Payments for Leasing Costs, Commissions, and Tenant Improvements						4,000
Interest Payments Deferred								16,711
Spread Over LIBOR														2.50%	2.50%	2.25%	2.25%	1.53%	1.36%
Debt Instrument, Description of Variable Rate Basis														LIBOR	LIBOR	LIBOR	LIBOR	Freddie Mac	LIBOR
Retained Proceeds From Refinancing														87,000		44,000
LIBOR Floor																			1.00%
Repayments of Convertible Debt																				$ 510,215