Debt (Details) (USD $)	0 Months Ended					6 Months Ended																																																																												6 Months Ended				6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended				6 Months Ended						6 Months Ended		6 Months Ended		6 Months Ended										6 Months Ended									6 Months Ended		6 Months Ended
	Jun. 10, 2011	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Retail Other [Member]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2011 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office2121 Crystal Drive [Member]	Dec. 31, 2010 Notes And Mortgages Payable Fixed Rate [Member] Washington Dc Office2121 Crystal Drive [Member]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Other Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Other Other [Member]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2011 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Dec. 31, 2010 Notes And Mortgages Payable Variable Rate [Member] Retail Other [Member]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2011 Total Notes And Mortgages Payable [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2011 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2011 [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2039 [Member]	Jun. 30, 2011 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Dec. 31, 2010 Senior Unsecured Notes [Member] Senior Unsecured Notes Due 2015 [Member]	Jun. 30, 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]	Jun. 30, 2011 Exchangeable Senior Debentures [Member] Exchangeable Senior Debenture 3.88% Due 2025 [Member]	Jun. 30, 2011 Convertible Senior Debentures [Member]	Jun. 30, 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]	Jun. 30, 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]	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member]	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.595 Billion [Member]	Dec. 31, 2010 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.595 Billion [Member]	Jun. 30, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion [Member]	Mar. 31, 2011 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion [Member]	Dec. 31, 2010 Unsecured Revolving Credit Facilities [Member] Unsecured Revolving Credit Facility $1.25 Billion [Member]
Debt Instrument Maturity								11/13		06/18		07/13		07/16		01/16		01/18		09/20				09/11		02/21		11/11		09/15		12/16		12/11		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		12/11		04/15		03/18		01/16		03/16		01/12		01/13				11/11		10/11		08/14		04/14		07/12		03/13		02/13		11/12		03/13		09/20		06/14		04/18		01/15						02/13		05/16		02/12						10/39		04/15		12/11		04/12		11/11		04/12			09/12		06/16
Maturity, Range, End						05/36
Maturity, Range, Start						03/12
Spread Over LIBOR	170.00%																																																																																	250.00%				300.00%		325.00%		350.00%		150.00%		140.00%		375.00%		400.00%		136.00%		140.00%				75.00%						120.00%		125.00%		55.00%										200.00%									55.00%		135.00%
Spread Over LIBOR Minimum																																																																																				235.00%
Spread Over LIBOR Maximum																																																																																				245.00%
Interest Rate, End of Period				559.00%				697.00%		529.00%		530.00%		604.00%		560.00%		459.00%		419.00%				579.00%		514.00%		695.00%		502.00%		557.00%		691.00%		708.00%		730.00%		726.00%		708.00%		675.00%		638.00%		708.00%		557.00%		614.00%		709.00%		543.00%		574.00%		461.00%		488.00%		551.00%		520.00%		564.00%		513.00%		571.00%		565.00%		548.00%		597.00%		217.00%		278.00%		258.00%		500.00%		352.00%		500.00%		177.00%		165.00%		394.00%		425.00%		236.00%		165.00%		153.00%		94.00%						145.00%		152.00%		74.00%		489.00%	590.00%			788.00%		425.00%		230.00%		532.00%	533.00%	532.00%		545.00%		72.00%	72.00%
Interest Rate, Range, Minimum						510.00%
Interest Rate, Range, Maximum						733.00%
Notes and mortgages payable		$ 8,575,022,000	$ 8,259,298,000	$ 6,808,543,000	$ 6,253,038,000	$ 100,476,000	$ 101,251,000	$ 56,912,000	$ 57,737,000	$ 80,000,000	$ 80,000,000	$ 88,994,000	$ 90,227,000	$ 120,000,000	$ 120,000,000	$ 31,961,000	$ 32,189,000	$ 75,000,000	$ 0	$ 591,327,000	$ 597,138,000	$ 0	$ 24,358,000	$ 642,163,000	$ 640,911,000	$ 60,000,000	$ 0	$ 43,021,000	$ 43,447,000	$ 67,947,000	$ 68,538,000	$ 550,000,000	$ 550,000,000	$ 5,162,000	$ 10,099,000	$ 27,044,000	$ 27,616,000	$ 81,005,000	$ 81,362,000	$ 44,734,000	$ 45,132,000	$ 45,211,000	$ 46,358,000	$ 51,815,000	$ 52,314,000	$ 101,182,000	$ 103,049,000	$ 78,142,000	$ 79,411,000	$ 93,000,000	$ 93,000,000	$ 115,022,000	$ 115,022,000	$ 109,891,000	$ 110,931,000	$ 195,546,000	$ 195,546,000	$ 678,000,000	$ 678,000,000	$ 75,704,000	$ 0	$ 101,671,000	$ 0	$ 150,000,000	$ 0	$ 197,260,000	$ 199,320,000	$ 205,142,000	$ 207,045,000	$ 425,000,000	$ 277,347,000	$ 318,554,000	$ 318,554,000	$ 353,000,000	$ 353,000,000	$ 430,000,000	$ 430,000,000	$ 418,657,000	$ 424,136,000	$ 1,766,479,000	$ 2,006,260,000	$ 22,400,000	$ 66,267,000	$ 123,750,000	$ 123,750,000	$ 51,603,000	$ 51,844,000	$ 75,000,000	$ 75,000,000	$ 100,000,000	$ 100,000,000	$ 279,044,000	$ 279,044,000	$ 325,045,000	$ 335,000,000	$ 21,733,000	$ 21,862,000	$ 117,025,000	$ 120,863,000	$ 60,000,000	$ 60,000,000	$ 43,581,000	$ 43,581,000	$ 64,000,000	$ 64,000,000	$ 56,320,000	$ 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
Senior unsecured notes		982,629,000	1,082,928,000																																																																																																																					0	100,382,000	460,000,000	460,000,000	499,379,000	499,296,000	23,250,000	23,250,000
Exchangeable senior debentures		494,403,000	491,000,000
Convertible senior debentures		187,994,000	186,413,000																																																																																																																															177,954,000	176,499,000	10,040,000	9,914,000
Interest Rate, Stated																																																																																																																																388.00%		363.00%		285.00%
Revolving credit facility debt		300,000,000	874,000,000																																																																																																																																				300,000,000	669,000,000	0		205,000,000
Line of Credit Facility, Maximum Borrowing Capacity																																																																																																																																							1,595,000,000		1,250,000,000	1,000,000,000
Line of Credit Facility, Commitment Fee Percentage																																																																																																																																									30.00%
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit																																																																																																																																									$ 21,534,000