Debt (Details) (USD $) In Thousands, unless otherwise specified																																																													9 Months Ended										9 Months Ended		0 Months Ended	9 Months Ended		9 Months Ended		1 Months Ended	9 Months Ended		9 Months Ended		9 Months Ended				9 Months Ended		9 Months Ended		9 Months Ended		9 Months Ended		9 Months Ended		9 Months Ended				9 Months Ended													9 Months Ended		9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 350 Park Avenue [Member]	Dec. 31, 2011 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 Two Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Two Penn Plaza [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 1290 Avenue Of Americas [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 1290 Avenue Of Americas [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 770 Broadway [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 770 Broadway [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 888 Seventh Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 888 Seventh Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 909 Third Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 828-850 Madison Avenue Condominium Retail [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 828-850 Madison Avenue Condominium Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] New York 510 5th Avenue Retail [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York 510 5th Avenue Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Skyline Properties [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC River House Apartments [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc 2121 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc 2121 Crystal Drive [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Bowen Building [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Bowen Building [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1215 Clark Street, 200 12th Street And 251 18th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1215 Clark Street, 200 12th Street And 251 18th Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC West End 25 [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC West End 25 [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Universal Buildings [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Universal Buildings [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2011 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2011 Crystal Drive [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1550 And 1750 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1550 And 1750 Crystal Drive [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc 220 20th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc 220 20th Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1235 Clark Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1235 Clark Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2231 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2231 Crystal Drive [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1225 Clark Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1225 Clark Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1750 Pennsylvania Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 1750 Pennsylvania Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2101 L Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC 2101 L Street [Member]	Sep. 30, 2012 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] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Montehiedra Town Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Montehiedra Town Center [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Broadway Mall [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Broadway Mall [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Las Catalinas Mall [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Las Catalinas Mall [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Merchandise Mart [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Merchandise Mart [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York Eleven Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Eleven Penn Plaza [Member]	Mar. 05, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York 100 West 33rd Street - Office & Retail [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 4 Union Square South - Retail [Member]	Dec. 31, 2011 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]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York 435 Seventh Avenue [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] New York 866 UN Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York 866 UN Plaza [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC River House Apartments [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC 2101 L Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC 2101 L Street [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC 2200/2300 Clarendon Boulevard [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC 2200/2300 Clarendon Boulevard [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Washington DC 1730 M And 1150 17th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC 1730 M And 1150 17th Street [Member]	Sep. 30, 2012 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 Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail San Jose Strip Centre [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail San Jose Strip Centre [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Sep. 30, 2012 Notes And Mortgages Payable Variable Rate [Member] Other 220 Central Park South [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Other 220 Central Park South [Member]	Sep. 30, 2012 Total Notes And Mortgages Payable [Member]	Sep. 30, 2012 Senior Unsecured Notes [Member]	Sep. 30, 2012 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Sep. 30, 2012 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Sep. 30, 2012 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Sep. 30, 2012 Exchangeable Senior Debentures [Member] Exchangeable Senior Debenture 3.88% [Member]	Sep. 30, 2012 Convertible Senior Debentures [Member] Convertible Senior Debenture 2.85% [Member]	Sep. 30, 2012 Unsecured Revolving Credit Facilities [Member]	Sep. 30, 2012 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]	Sep. 30, 2012 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 06/16 [Member]
Debt Instrument Maturity					01/17		03/18		01/13		03/16		01/16		04/15		06/18		01/16		02/17		04/15		03/23		06/16		01/25		06/21		04/14		08/17		11/14		02/18				08/13		08/13				08/24		09/20		07/16		07/13		01/18		11/13				12/16		09/21		02/21				01/19			03/17		04/14			08/19		05/16		04/18				01/15		06/14		09/20		11/12		03/13		03/13				10/13				04/15		10/39		01/22					11/16		06/16
Maturity, Range, Start																																																													06/14
Maturity, Range, End																																																													05/36
Spread Over LIBOR																																																																							2.35%		2.50%	2.50%		3.25%		2.25%	2.25%		1.25%		1.53%				0.75%		1.40%		1.36%		3.75%		1.50%		4.00%				2.75%													1.25%		1.35%
Debt Instrument, Description of Variable Rate Basis																																																																							LIBOR		LIBOR	LIBOR		LIBOR		LIBOR	LIBOR		LIBOR		Freddie Mac				LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR				LIBOR													LIBOR		LIBOR
Interest Rate, End of Period			5.40%		3.75%		5.13%		5.97%		5.65%		5.71%		5.64%		5.29%		5.60%		5.74%		5.43%		5.51%		6.14%		7.09%		4.88%		6.49%		7.30%		7.08%		4.61%				7.08%		7.08%				3.97%		4.23%		6.04%		5.30%		4.59%		6.97%				5.57%		5.10%		5.14%		2.50%		2.58%			2.73%		3.48%			2.47%		1.48%		1.63%				0.98%		1.62%		2.36%		3.98%		1.73%		4.25%				2.97%		4.80%	5.70%	4.25%		7.88%		5.00%				1.43%	1.43%
Interest Rate, Range, Minimum																																																													5.12%
Interest Rate, Range, Maximum																																																													7.30%
Notes and mortgages payable	$ 7,852,657	$ 8,065,576	$ 6,231,418	$ 6,321,628	$ 300,000	$ 430,000	$ 425,000	$ 425,000	$ 410,021	$ 413,111	$ 353,000	$ 353,000	$ 318,554	$ 318,554	$ 200,241	$ 203,217	$ 80,000	$ 80,000	$ 31,377	$ 31,732	$ 694,711	$ 678,000	$ 195,546	$ 195,546	$ 150,000	$ 150,000	$ 115,022	$ 115,022	$ 106,628	$ 108,423	$ 101,671	$ 101,671	$ 94,497	$ 98,239	$ 79,865	$ 80,486	$ 74,765	$ 76,624	$ 74,246	$ 75,037	$ 0	$ 51,309	$ 42,160	$ 43,819	$ 25,219	$ 26,211	$ 0	$ 44,330	$ 150,000	$ 0	$ 576,281	$ 585,398	$ 120,000	$ 120,000	$ 85,840	$ 87,750	$ 75,000	$ 75,000	$ 54,719	$ 55,912	$ 87,055	$ 88,237	$ 550,000	$ 550,000	$ 600,000	$ 600,000	$ 60,000	$ 60,000	$ 1,621,239	$ 1,743,948	$ 330,000	$ 330,000		$ 325,000	$ 232,000	$ 75,000	$ 75,000		$ 98,000	$ 51,353	$ 44,978	$ 44,978	$ 64,000	$ 64,000	$ 0	$ 150,000	$ 48,859	$ 53,344	$ 43,581	$ 43,581	$ 60,000	$ 60,000	$ 19,427	$ 19,876	$ 282,312	$ 283,590	$ 106,332	$ 112,476	$ 0	$ 100,000	$ 123,750	$ 123,750
Senior unsecured notes	1,357,921	1,357,661																																																																																																							499,586	499,462	460,000	460,000	398,335	398,199
Exchangeable senior debentures	0	497,898
Convertible senior debentures	0	10,168
Interest Rate, Stated																																																																																																															3.88%	2.85%
Revolving credit facility debt	600,000	138,000																																																																																																																600,000	138,000	0	0
Line of Credit Facility, Maximum Borrowing Capacity																																																																																																																		1,250,000		1,250,000
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit																																																																																																																				$ 22,576