Debt (Details) (USD $)							0 Months Ended									12 Months Ended																																																												0 Months Ended									12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended						12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended																							12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office 350 Park Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office 350 Park Avenue [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. 31, 2010 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 1290 Avenue Of Americas [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office 1290 Avenue Of Americas [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office 770 Broadway [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office 770 Broadway [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office 888 Seventh Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office 888 Seventh Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Las Catalinas Mall [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail Las Catalinas Mall [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail 828-850 Madison Avenue Condominium [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail 828-850 Madison Avenue Condominium [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Broadway Mall [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail Broadway Mall [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Montehiedra Town Center [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail Montehiedra Town Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail 510 5th Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail 510 5th Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail North Bergen Tonnelle Avenue [Member]	Dec. 31, 2010 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, 2010 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 Industrial Warehouses [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Other Industrial Warehouses [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Other 555 California Street [Member]	Dec. 31, 2010 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]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Other Borgata Land [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Washington Design Center [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Washington Design Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Boston Design Center [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Boston Design Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Merchandise Mart [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Merchandise Mart Merchandise Mart [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1800, 1851 And 1901 South Bell Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1800, 1851 And 1901 South Bell Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1225 Clark Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1225 Clark Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 2011 Crystal Drive [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 2011 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1750 Pennsylvania Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1750 Pennsylvania Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 2231 Crystal Drive [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 2231 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1235 Clark Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1235 Clark Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Universal Buildings [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Universal Buildings [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1550 And 1750 Crystal Drive [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1550 And 1750 Crystal Drive [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Reston Executive I, II, And III [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Reston Executive I, II, And III [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Bowen Building [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Bowen Building [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1215 Clark Street, 200 12th Street And 251 18th Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office 1215 Clark Street, 200 12th Street And 251 18th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington DC Office Skyline Place [Member]	Dec. 31, 2010 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 220 20th Street [Member]	Dec. 31, 2010 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 West End 25 [Member]	Dec. 31, 2010 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 2121 Crystal Drive [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office 2121 Crystal Drive [Member]	Dec. 28, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Fixed Rate [Member] New York Office 909 Third Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] New York Office 909 Third Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Other Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Other Other [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Other 220 Central Park South [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Other 220 Central Park South [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail 435 Seventh Avenue [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail 435 Seventh Avenue [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail 4 Union Square South [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail 4 Union Square South [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail Beverly Connection [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail Bergen Town Center [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Green Acres Mall [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail Green Acres Mall [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail San Jose Strip Centre [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail San Jose Strip Centre [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]	Dec. 31, 2010 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] Washington DC Office 1730 M And 1150 17th Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office 1730 M And 1150 17th Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office River House Apartments [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC 2200/2300 Clarendon Boulevard [Member]	Dec. 31, 2010 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 Office 220 20th Street [Member]	Dec. 31, 2010 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 West End 25 [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office West End 25 [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office 2101 L Street [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Washington DC Office 2101 L Street [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office 866 UN Plaza [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] New York Office 866 UN Plaza [Member]	Dec. 31, 2011 Notes And Mortgages Payable Variable Rate [Member] New York Office Manhattan Mall [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] New York Office Manhattan Mall [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, 2010 Notes And Mortgages Payable Variable Rate [Member] New York Office Eleven Penn Plaza [Member]	Dec. 31, 2011 Total Notes And Mortgages Payable [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2011 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2011 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Floating Rate Senior Unsecured Notes Due 2011 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Floating Rate Senior Unsecured Notes Due 2011 [Member]	Dec. 31, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Nov. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2022 [Member]	Dec. 31, 2011 Exchangeable Senior Debentures [Member] Exchangeable Senior Debenture 3.88% Due 2025 [Member]	Dec. 31, 2011 Convertible Senior Debentures [Member]	Dec. 31, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 3.63% Due 2026 [Member]	Dec. 31, 2010 Convertible Senior Debentures [Member] Convertible Senior Debenture 3.63% Due 2026 [Member]	Dec. 31, 2011 Convertible Senior Debentures [Member] Convertible Senior Debenture 2.85% Due 2027 [Member]	Dec. 31, 2010 Convertible Senior Debentures [Member] Convertible Senior Debenture 2.85% Due 2027 [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member]	Dec. 31, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]	Dec. 31, 2010 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 06/16 [Member]	Dec. 31, 2010 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]	Dec. 31, 2011 Mortgages Payable [Member]	Dec. 31, 2011 Senior Unsecured Debt And Revolving Credit Facilities [Member]
Debt Repayment Of Principal [Line Items]
2012																																																																																																																																																		$ 828,404,000	$ 0
2013																																																																																																																																																		1,741,750,000	0
2014																																																																																																																																																		495,098,000	0
2015																																																																																																																																																		538,159,000	500,000,000
2016																																																																																																																																																		1,576,394,000	138,000,000
Thereafter																																																																																																																																																		3,366,770,000	860,000,000
Statement [Line Items]
Debt Instrument Maturity					01/12			03/18		01/13		03/16		01/16				11/13		06/18		07/13		07/16		01/16		01/18		09/20				09/21		02/21				09/15		12/16				08/13		08/17		06/12		08/13		07/12		04/14		11/14		01/13		06/16		01/25		04/15		02/17		02/18		06/21		03/23					04/15						10/13		08/14		04/14		09/14		03/13		02/13		11/12		03/13		09/20		06/14		04/18		01/15						02/13		05/16		02/12		01/19							10/39		04/15				01/22			04/12				04/12			11/16		06/16
Maturity, Range, End																05/36
Maturity, Range, Start																03/12
Spread Over LIBOR							2.00%																																																																														2.75%		3.00%		3.25%		4.25%		1.50%		1.40%		3.75%		4.00%		1.36%		1.40%		1.53%		0.75%						1.20%		1.25%		0.55%		2.35%	2.35%																						1.25%		1.35%
Debt Instrument, Description of Variable Rate Basis							LIBOR																																																																					LIBOR									LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		LIBOR		Freddie Mac		LIBOR						LIBOR		LIBOR		LIBOR		LIBOR																							LIBOR		LIBOR
Interest Rate, End of Period			5.53%		5.48%			5.13%		5.97%		5.65%		5.71%				6.97%		5.29%		5.30%		6.04%		5.60%		4.59%		4.21%				5.10%		5.14%				5.02%		5.57%				7.08%		7.30%		7.26%		7.08%		6.75%		6.44%		7.08%		5.57%		6.14%		7.09%		5.43%		5.74%		4.61%		4.88%		5.51%					5.64%		2.30%				3.03%		5.00%		3.69%		4.75%		1.93%		1.67%		4.02%		4.32%		2.36%		1.69%		1.53%		1.03%						1.50%		1.69%		0.83%		2.60%			4.75%	5.70%			7.88%		4.25%				5.00%			5.32%	5.45%			5.45%		1.48%	1.48%
Interest Rate, Range, Minimum																5.12%
Interest Rate, Range, Maximum																7.30%
Notes and mortgages payable	8,558,275,000	8,255,101,000	6,489,282,000	6,248,841,000	430,000,000	430,000,000		425,000,000	277,347,000	413,111,000	424,136,000	353,000,000	353,000,000	318,554,000	318,554,000	95,541,000	97,054,000	55,912,000	57,737,000	80,000,000	80,000,000	87,750,000	90,227,000	120,000,000	120,000,000	31,732,000	32,189,000	75,000,000	0	585,398,000	597,138,000	0	24,358,000	600,000,000	640,911,000	60,000,000	0	0	43,447,000	67,350,000	68,538,000	550,000,000	550,000,000	0	10,099,000	26,211,000	27,616,000	80,486,000	81,362,000	44,330,000	45,132,000	43,819,000	46,358,000	51,309,000	52,314,000	98,239,000	103,049,000	76,624,000	79,411,000	93,000,000	93,000,000	115,022,000	115,022,000	108,423,000	110,931,000	195,546,000	195,546,000	678,000,000	678,000,000	75,037,000	0	101,671,000	0	150,000,000	0		0	199,320,000	203,217,000	207,045,000	2,068,993,000	2,006,260,000	0	66,267,000	123,750,000	123,750,000	51,353,000	51,844,000	75,000,000	75,000,000	100,000,000	100,000,000	283,590,000	279,044,000	325,045,000	335,000,000	19,876,000	21,862,000	112,476,000	120,863,000	60,000,000	60,000,000	43,581,000	43,581,000	64,000,000	64,000,000	53,344,000	59,278,000	0	83,573,000	0	95,220,000	150,000,000	150,000,000	44,978,000	44,978,000	232,000,000	232,000,000	330,000,000		0											398,199,000		0
Senior unsecured notes	1,357,661,000	1,082,928,000																																																																																																																										0	100,382,000	460,000,000	460,000,000	499,462,000	499,296,000	0	23,250,000
Exchangeable senior debentures	497,898,000	491,000,000
Convertible senior debentures	10,168,000	186,413,000																																																																																																																																							0	176,499,000	10,168,000	9,914,000
Interest Rate, Stated																																																																																																																																					5.00%		3.88%		3.63%		2.85%
Revolving credit facility debt	138,000,000	874,000,000																																																																																																																																												138,000,000	669,000,000	0	205,000,000
Collateral Property Net Carrying Value	10,400,000,000
Line of Credit Facility, Maximum Borrowing Capacity	2,500,000,000																																																																																																																																													1,250,000,000		1,250,000,000
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit																																																																																																																																																$ 22,085,000