Debt (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended				0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended				0 Months Ended		0 Months Ended		3 Months Ended			0 Months Ended				1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 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	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member]	Nov. 16, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 4 Union Square South - Retail [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 4 Union Square South - Retail [Member]	Aug. 17, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member]	Nov. 08, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 1290 Avenue Of Americas [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 1290 Avenue Of Americas [Member]	Jan. 09, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 350 Park Avenue [Member]	Dec. 31, 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]	Dec. 31, 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]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2101 L Street [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Apr. 30, 2012 Exchangeable And Convertible Senior Debentures [Member]
Debt Instrument [Line Items]
Spread Over LIBOR						2.50%		2.15%		2.25%	1.53%	1.36%
Debt Instrument, Description of Variable Rate Basis						LIBOR		LIBOR		LIBOR	Freddie Mac	LIBOR
Interest Rate, End of Period				2.22%		2.71%		2.36%		2.46%	1.63%	2.36%	5.07%		3.34%		3.75%			5.74%		3.97%	5.43%	4.23%
LIBOR Floor												1.00%
Notes And Loans Payable Refinanced Amount					$ 325,000		$ 120,000		$ 98,000					$ 950,000		$ 300,000					$ 150,000
Retained Proceeds From Refinancing					87,000		42,000		44,000					522,000
Debt Amortization Period							30 years									30 years					30 years
Vacancy Rate																			26.00%
Repayments of borrowings	2,747,694	3,740,327	1,564,143													132,000
Debt Term					3 years		7 years		7 years					10 years		5 years					12 years
Number Of Extensions Available					2
Length Of Extension Available In Years					1 year
Payments for Leasing Costs, Commissions, and Tenant Improvements																		4,000
Interest Payments Deferred																				26,957
Repayments of Convertible Debt																									$ 510,215