Debt (Parentheticals) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended				12 Months Ended			12 Months Ended		12 Months Ended			12 Months Ended					0 Months Ended			0 Months Ended																		0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 28, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office 866 UN Plaza [Member]	May 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office West End 25 [Member]	Jan. 18, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office 220 20th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail 435 Seventh Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Jan. 09, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York Office 350 Park Avenue New Loan [Member]	Feb. 11, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Two Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Two Penn Plaza [Member]	Dec. 28, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Eleven Penn Plaza [Member]	Feb. 06, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Skyline Place [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Skyline Place [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office 2121 Crystal Drive [Member]	Feb. 10, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office 2121 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office West End 25 [Member]	May 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office West End 25 [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 220 20th Street [Member]	Jan. 18, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 220 20th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Jan. 10, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Sep. 01, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Jan. 06, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member]	Nov. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]
Spread Over LIBOR					1.35%	1.25%		2.35%	2.35%	1.25%			1.53%	4.25%	1.36%	3.00%			2.00%
Debt Instrument, Description of Variable Rate Basis					LIBOR	LIBOR		LIBOR		LIBOR			Freddie Mac	LIBOR	LIBOR	LIBOR			LIBOR		LIBOR
Interest Rate, Stated																																							5.00%
Interest Rate, Effective	5.45%																																						5.057%
Interest Rate, End of Period				1.48%		1.48%	2.30%	2.60%		1.69%			1.53%	4.75%	2.36%	5.00%	5.53%	3.75%		5.13%			5.74%	5.51%		4.88%		4.61%		5.43%	4.59%		4.21%	5.10%		5.14%		5.70%		5.00%
LIBOR Floor														0.50%	1.00%	2.00%
Line of Credit Facility, Maximum Borrowing Capacity	$ 2,500,000				$ 1,250,000	$ 1,250,000
Notes And Loans Payable Refinanced Amount									330,000									300,000	425,000																600,000
Notes And Loans Payable Financing Amount											101,671	76,100													150,000							75,000					60,000
Retained Proceeds From Refinancing																			139,000
Debt Amortization Period									30									30	30						30		30		30			25			30		30
Line of Credit Facility, Commitment Fee Percentage					0.30%	0.25%
Vacancy Rate																						26.00%
Repayments of borrowings	3,740,327	1,564,143	2,075,236															132,000
Debt Instrument, Face Amount																																							400,000
Proceeds from borrowings	$ 3,412,897	$ 2,481,883	$ 2,648,175																																				$ 395,584
Selling Price As Percentage Of Face Value																																							99.546%