Debt (Details) (USD $) In Thousands, unless otherwise specified	0 Months Ended	3 Months Ended
	Mar. 05, 2012	Mar. 31, 2012	Dec. 31, 2011
Notes and mortgages payable		$ 8,434,938	$ 8,558,275
Senior unsecured notes		1,357,748	1,357,661
Exchangeable senior debentures		499,680	497,898
Convertible senior debentures		10,233	10,168
Revolving credit facility debt		0	138,000
Notes And Mortgages Payable Fixed Rate [Member]
Interest Rate, End of Period		5.44%
Notes and mortgages payable		6,295,942	6,489,282
Notes And Mortgages Payable Fixed Rate [Member] | New York 350 Park Avenue [Member]
Debt Instrument Maturity		01/17
Interest Rate, End of Period		3.75%
Notes and mortgages payable		300,000	430,000
Notes And Mortgages Payable Fixed Rate [Member] | New York Two Penn Plaza [Member]
Debt Instrument Maturity		03/18
Interest Rate, End of Period		5.13%
Notes and mortgages payable		425,000	425,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 1290 Avenue Of Americas [Member]
Debt Instrument Maturity		01/13
Interest Rate, End of Period		5.97%
Notes and mortgages payable		411,661	413,111
Notes And Mortgages Payable Fixed Rate [Member] | New York 770 Broadway [Member]
Debt Instrument Maturity		03/16
Interest Rate, End of Period		5.65%
Notes and mortgages payable		353,000	353,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 888 Seventh Avenue [Member]
Debt Instrument Maturity		01/16
Interest Rate, End of Period		5.71%
Notes and mortgages payable		318,554	318,554
Notes And Mortgages Payable Fixed Rate [Member] | New York 909 Third Avenue [Member]
Debt Instrument Maturity		04/15
Interest Rate, End of Period		5.64%
Notes and mortgages payable		202,218	203,217
Notes And Mortgages Payable Fixed Rate [Member] | New York 828-850 Madison Avenue Condominium Retail [Member]
Debt Instrument Maturity		06/18
Interest Rate, End of Period		5.29%
Notes and mortgages payable		80,000	80,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 510 5th Avenue Retail [Member]
Debt Instrument Maturity		01/16
Interest Rate, End of Period		5.60%
Notes and mortgages payable		31,612	31,732
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Skyline Place [Member]
Debt Instrument Maturity		02/17
Interest Rate, End of Period		5.74%
Notes and mortgages payable		678,000	678,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC River House Apartments [Member]
Debt Instrument Maturity		04/15
Interest Rate, End of Period		5.43%
Notes and mortgages payable		195,546	195,546
Notes And Mortgages Payable Fixed Rate [Member] | Washington Dc 2121 Crystal Drive [Member]
Debt Instrument Maturity		03/23
Interest Rate, End of Period		5.51%
Notes and mortgages payable		150,000	150,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Bowen Building [Member]
Debt Instrument Maturity		06/16
Interest Rate, End of Period		6.14%
Notes and mortgages payable		115,022	115,022
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1215 Clark Street, 200 12th Street And 251 18th Street [Member]
Debt Instrument Maturity		01/25
Interest Rate, End of Period		7.09%
Notes and mortgages payable		107,766	108,423
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC West End 25 [Member]
Debt Instrument Maturity		06/21
Interest Rate, End of Period		4.88%
Notes and mortgages payable		101,671	101,671
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Universal Buildings [Member]
Debt Instrument Maturity		04/14
Interest Rate, End of Period		6.45%
Notes and mortgages payable		97,003	98,239
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Reston Executive I, II, And III [Member]
Debt Instrument Maturity		01/13
Interest Rate, End of Period		5.57%
Notes and mortgages payable		93,000	93,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2011 Crystal Drive [Member]
Debt Instrument Maturity		08/17
Interest Rate, End of Period		7.30%
Notes and mortgages payable		80,256	80,486
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1550 And 1750 Crystal Drive [Member]
Debt Instrument Maturity		11/14
Interest Rate, End of Period		7.08%
Notes and mortgages payable		75,946	76,624
Notes And Mortgages Payable Fixed Rate [Member] | Washington Dc 220 20th Street [Member]
Debt Instrument Maturity		02/18
Interest Rate, End of Period		4.61%
Notes and mortgages payable		74,739	75,037
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1235 Clark Street [Member]
Debt Instrument Maturity		07/12
Interest Rate, End of Period		6.75%
Notes and mortgages payable		51,045	51,309
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2231 Crystal Drive [Member]
Debt Instrument Maturity		08/13
Interest Rate, End of Period		7.08%
Notes and mortgages payable		43,205	43,819
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1225 Clark Street [Member]
Debt Instrument Maturity		08/13
Interest Rate, End of Period		7.08%
Notes and mortgages payable		25,844	26,211
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1750 Pennsylvania Avenue [Member]
Notes and mortgages payable		0	44,330
Notes And Mortgages Payable Fixed Rate [Member] | Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]
Debt Instrument Maturity		09/20
Interest Rate, End of Period		4.21%
Notes and mortgages payable		582,389	585,398
Notes And Mortgages Payable Fixed Rate [Member] | Retail Montehiedra Town Center [Member]
Debt Instrument Maturity		07/16
Interest Rate, End of Period		6.04%
Notes and mortgages payable		120,000	120,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Broadway Mall [Member]
Debt Instrument Maturity		07/13
Interest Rate, End of Period		5.30%
Notes and mortgages payable		87,111	87,750
Notes And Mortgages Payable Fixed Rate [Member] | Retail North Bergen Tonnelle Avenue [Member]
Debt Instrument Maturity		01/18
Interest Rate, End of Period		4.59%
Notes and mortgages payable		75,000	75,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Las Catalinas Mall [Member]
Debt Instrument Maturity		11/13
Interest Rate, End of Period		6.97%
Notes and mortgages payable		55,471	55,912
Notes And Mortgages Payable Fixed Rate [Member] | Retail Other [Member]
Maturity, Range, Start		06/14
Maturity, Range, End		05/36
Interest Rate, Range, Minimum		5.12%
Interest Rate, Range, Maximum		7.30%
Notes and mortgages payable		87,841	95,541
Notes And Mortgages Payable Fixed Rate [Member] | Merchandise Mart Merchandise Mart [Member]
Debt Instrument Maturity		12/16
Interest Rate, End of Period		5.57%
Notes and mortgages payable		550,000	550,000
Notes And Mortgages Payable Fixed Rate [Member] | Merchandise Mart Boston Design Center [Member]
Debt Instrument Maturity		09/15
Interest Rate, End of Period		5.02%
Notes and mortgages payable		67,042	67,350
Notes And Mortgages Payable Fixed Rate [Member] | Other 555 California Street [Member]
Debt Instrument Maturity		09/21
Interest Rate, End of Period		5.10%
Notes and mortgages payable		600,000	600,000
Notes And Mortgages Payable Fixed Rate [Member] | Other Borgata Land [Member]
Debt Instrument Maturity		02/21
Interest Rate, End of Period		5.14%
Notes and mortgages payable		60,000	60,000
Notes And Mortgages Payable Variable Rate [Member]
Interest Rate, End of Period		2.48%
Notes and mortgages payable		2,138,996	2,068,993
Notes And Mortgages Payable Variable Rate [Member] | New York Eleven Penn Plaza [Member]
Debt Instrument Maturity		01/19
Spread Over LIBOR		2.35%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		2.59%
Notes and mortgages payable		330,000	330,000
Notes And Mortgages Payable Variable Rate [Member] | New York 100 West 33rd Street - Office & Retail [Member]
Debt Instrument Maturity		03/17
Spread Over LIBOR	2.50%	2.50%
Debt Instrument, Description of Variable Rate Basis	LIBOR	LIBOR
Interest Rate, End of Period		2.74%
Notes and mortgages payable		325,000	232,000
Notes And Mortgages Payable Variable Rate [Member] | New York 4 Union Square South - Retail [Member]
Debt Instrument Maturity		04/14
Spread Over LIBOR		3.25%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		3.49%
Notes and mortgages payable		75,000	75,000
Notes And Mortgages Payable Variable Rate [Member] | New York 435 Seventh Avenue [Member]
Debt Instrument Maturity		08/14
Spread Over LIBOR		3.00%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		5.00%
Notes and mortgages payable		51,224	51,353
Notes And Mortgages Payable Variable Rate [Member] | New York 866 UN Plaza [Member]
Debt Instrument Maturity		05/16
Spread Over LIBOR		1.25%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		1.49%
Notes and mortgages payable		44,978	44,978
Notes And Mortgages Payable Variable Rate [Member] | Washington DC River House Apartments [Member]
Debt Instrument Maturity		04/18
Spread Over LIBOR		1.53%
Debt Instrument, Description of Variable Rate Basis		Freddie Mac
Interest Rate, End of Period		1.60%
Notes and mortgages payable		64,000	64,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 2101 L Street [Member]
Debt Instrument Maturity		02/13
Spread Over LIBOR		1.20%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		1.44%
Notes and mortgages payable		148,125	150,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 2200/2300 Clarendon Boulevard [Member]
Debt Instrument Maturity		01/15
Spread Over LIBOR		0.75%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		0.99%
Notes and mortgages payable		51,856	53,344
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 1730 M And 1150 17th Street [Member]
Debt Instrument Maturity		06/14
Spread Over LIBOR		1.40%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		1.64%
Notes and mortgages payable		43,581	43,581
Notes And Mortgages Payable Variable Rate [Member] | Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]
Debt Instrument Maturity		09/20
Spread Over LIBOR		1.36%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		2.36%
Notes and mortgages payable		60,000	60,000
Notes And Mortgages Payable Variable Rate [Member] | Retail Other [Member]
Debt Instrument Maturity		11/12
Spread Over LIBOR		3.75%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		3.99%
Notes and mortgages payable		19,726	19,876
Notes And Mortgages Payable Variable Rate [Member] | Retail Green Acres Mall [Member]
Debt Instrument Maturity		02/13
Spread Over LIBOR		1.40%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		1.65%
Notes and mortgages payable		308,825	325,045
Notes And Mortgages Payable Variable Rate [Member] | Retail Bergen Town Center [Member]
Debt Instrument Maturity		03/13
Spread Over LIBOR		1.50%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		1.74%
Notes and mortgages payable		282,312	283,590
Notes And Mortgages Payable Variable Rate [Member] | Retail San Jose Strip Centre [Member]
Debt Instrument Maturity		03/13
Spread Over LIBOR		4.00%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		4.25%
Notes and mortgages payable		110,619	112,476
Notes And Mortgages Payable Variable Rate [Member] | Retail Beverly Connection [Member]
Debt Instrument Maturity		09/14
Spread Over LIBOR		4.25%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		4.75%
Notes and mortgages payable		100,000	100,000
Notes And Mortgages Payable Variable Rate [Member] | Other 220 Central Park South [Member]
Debt Instrument Maturity		10/13
Spread Over LIBOR		2.75%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Interest Rate, End of Period		2.99%
Notes and mortgages payable		123,750	123,750
Total Notes And Mortgages Payable [Member]
Interest Rate, End of Period		4.69%
Senior Unsecured Notes [Member]
Interest Rate, End of Period		5.70%
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2015 [Member]
Debt Instrument Maturity		04/15
Interest Rate, End of Period		4.25%
Senior unsecured notes		499,503	499,462
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2039 [Member]
Debt Instrument Maturity		10/39
Interest Rate, End of Period		7.88%
Senior unsecured notes		460,000	460,000
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2022 [Member]
Debt Instrument Maturity		01/22
Interest Rate, End of Period		5.00%
Senior unsecured notes		398,245	398,199
Exchangeable Senior Debentures [Member] | Exchangeable Senior Debenture 3.88% [Member]
Debt Instrument Maturity		04/12
Interest Rate, End of Period		5.32%
Interest Rate, Stated		3.88%
Convertible Senior Debentures [Member] | Convertible Senior Debenture 2.85% [Member]
Debt Instrument Maturity		04/12
Interest Rate, End of Period		5.45%
Interest Rate, Stated		2.85%
Unsecured Revolving Credit Facilities [Member] | Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]
Debt Instrument Maturity		11/16
Spread Over LIBOR		1.25%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Revolving credit facility debt		0	138,000
Line of Credit Facility, Maximum Borrowing Capacity		1,250,000
Unsecured Revolving Credit Facilities [Member] | Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]
Debt Instrument Maturity		06/16
Spread Over LIBOR		1.35%
Debt Instrument, Description of Variable Rate Basis		LIBOR
Revolving credit facility debt		0	0
Line of Credit Facility, Maximum Borrowing Capacity		1,250,000
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit		$ 22,085