Debt (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Mortgages payable	$ 8,768,182,000	$ 8,072,880,000
Senior unsecured notes	1,358,008,000	1,357,661,000
Exchangeable senior debentures	0	497,898,000
Convertible senior debentures	0	10,168,000
Revolving credit facility debt	1,170,000,000	138,000,000
Collateral Property Net Carrying Value	10,400,000,000
Notes And Mortgages Payable Fixed Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	5.07%
Mortgages payable	6,771,001,000	6,328,932,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 1290 Avenue Of Americas [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/22
Interest Rate, End of Period	3.34%
Mortgages payable	950,000,000	413,111,000
Notes And Mortgages Payable Fixed Rate [Member] | New York Two Penn Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/18
Interest Rate, End of Period	5.13%
Mortgages payable	425,000,000	425,000,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 770 Broadway [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/16
Interest Rate, End of Period	5.65%
Mortgages payable	353,000,000	353,000,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 888 Seventh Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/16
Interest Rate, End of Period	5.71%
Mortgages payable	318,554,000	318,554,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 350 Park Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/17
Interest Rate, End of Period	3.75%
Mortgages payable	300,000,000	430,000,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 909 Third Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/15
Interest Rate, End of Period	5.64%
Mortgages payable	199,198,000	203,217,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 828-850 Madison Avenue Condominium - Retail [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/18
Interest Rate, End of Period	5.29%
Mortgages payable	80,000,000	80,000,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 510 5th Avenue - Retail [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/16
Interest Rate, End of Period	5.60%
Mortgages payable	31,253,000	31,732,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Skyline Properties [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	02/17
Interest Rate, End of Period	5.74%
Mortgages payable	704,957,000	678,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC River House Apartments [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/15
Interest Rate, End of Period	5.43%
Mortgages payable	195,546,000	195,546,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2101 L Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/24
Interest Rate, End of Period	3.97%
Mortgages payable	150,000,000	0
Notes And Mortgages Payable Fixed Rate [Member] | Washington Dc 2121 Crystal Drive [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/23
Interest Rate, End of Period	5.51%
Mortgages payable	150,000,000	150,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Bowen Building [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/16
Interest Rate, End of Period	6.14%
Mortgages payable	115,022,000	115,022,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1215 Clark Street, 200 12th Street And 251 18th Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/25
Interest Rate, End of Period	7.09%
Mortgages payable	105,724,000	108,423,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC West End 25 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/21
Interest Rate, End of Period	4.88%
Mortgages payable	101,671,000	101,671,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC Universal Buildings [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/14
Interest Rate, End of Period	6.50%
Mortgages payable	93,226,000	98,239,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2011 Crystal Drive [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/17
Interest Rate, End of Period	7.30%
Mortgages payable	79,624,000	80,486,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1550 And 1750 Crystal Drive [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/14
Interest Rate, End of Period	7.08%
Mortgages payable	74,053,000	76,624,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 220 20th Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	02/18
Interest Rate, End of Period	4.61%
Mortgages payable	73,939,000	75,037,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2231 Crystal Drive [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/13
Interest Rate, End of Period	7.08%
Mortgages payable	41,298,000	43,819,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1225 Clark Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/13
Interest Rate, End of Period	7.08%
Mortgages payable	24,834,000	26,211,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1235 Clark Street [Member]
Debt Instrument [Line Items]
Mortgages payable	0	51,309,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1750 Pennsylvania Avenue [Member]
Debt Instrument [Line Items]
Mortgages payable	0	44,330,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	09/20
Interest Rate, End of Period	4.23%
Mortgages payable	573,180,000	585,398,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Montehiedra Town Center [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	07/16
Interest Rate, End of Period	6.04%
Mortgages payable	120,000,000	120,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Broadway Mall [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	07/13
Interest Rate, End of Period	5.30%
Mortgages payable	85,180,000	87,750,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail North Bergen Tonnelle Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/18
Interest Rate, End of Period	4.59%
Mortgages payable	75,000,000	75,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Las Catalinas Mall [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/13
Interest Rate, End of Period	6.97%
Mortgages payable	54,101,000	55,912,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Maturity, Range, End	05/36
Maturity, Range, Start	06/14
Interest Rate, Range, Minimum	5.12%
Interest Rate, Range, Maximum	7.30%
Mortgages payable	86,641,000	95,541,000
Notes And Mortgages Payable Fixed Rate [Member] | Merchandise Mart Merchandise Mart [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	12/16
Interest Rate, End of Period	5.57%
Mortgages payable	550,000,000	550,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Other 555 California Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	09/21
Interest Rate, End of Period	5.10%
Mortgages payable	600,000,000	600,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Other Borgata Land [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	02/21
Interest Rate, End of Period	5.14%
Mortgages payable	60,000,000	60,000,000
Notes And Mortgages Payable Variable Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	2.22%
Mortgages payable	1,997,181,000	1,743,948,000
Notes And Mortgages Payable Variable Rate [Member] | New York Independence Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/13
Spread Over LIBOR	0.92%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.15%
Mortgages payable	334,225,000	0
Notes And Mortgages Payable Variable Rate [Member] | New York Eleven Penn Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/19
Spread Over LIBOR	2.35%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	2.56%
Mortgages payable	330,000,000	330,000,000
Notes And Mortgages Payable Variable Rate [Member] | New York 100 West 33rd Street - Office & Retail [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/17
Spread Over LIBOR	2.50%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	2.71%
Mortgages payable	325,000,000	232,000,000
Notes And Mortgages Payable Variable Rate [Member] | New York 4 Union Square South - Retail [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/19
Spread Over LIBOR	2.15%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	2.36%
Mortgages payable	120,000,000	75,000,000
Notes And Mortgages Payable Variable Rate [Member] | New York 435 Seventh Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/19
Spread Over LIBOR	2.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	2.46%
Mortgages payable	98,000,000	51,353,000
Notes And Mortgages Payable Variable Rate [Member] | New York 866 UN Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	05/16
Spread Over LIBOR	1.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.46%
Mortgages payable	44,978,000	44,978,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC River House Apartments [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/18
Spread Over LIBOR	1.53%
Debt Instrument, Description of Variable Rate Basis	Freddie Mac
Interest Rate, End of Period	1.63%
Mortgages payable	64,000,000	64,000,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 2101 L Street [Member]
Debt Instrument [Line Items]
Mortgages payable	0	150,000,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 2200/2300 Clarendon Boulevard [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/15
Spread Over LIBOR	0.75%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	0.96%
Mortgages payable	47,353,000	53,344,000
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 1730 M And 1150 17th Street [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/14
Spread Over LIBOR	1.40%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.61%
Mortgages payable	43,581,000	43,581,000
Notes And Mortgages Payable Variable Rate [Member] | Retail Cross Collateralized Mortgages On 40 Strip Shopping Centers [Member]
Debt Instrument [Line Items]
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%
Mortgages payable	60,000,000	60,000,000
Notes And Mortgages Payable Variable Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/13
Spread Over LIBOR	3.75%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	3.97%
Mortgages payable	19,126,000	19,876,000
Notes And Mortgages Payable Variable Rate [Member] | Retail Bergen Town Center [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/13
Spread Over LIBOR	1.50%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.71%
Mortgages payable	282,312,000	283,590,000
Notes And Mortgages Payable Variable Rate [Member] | Retail San Jose Strip Center [Member]
Debt Instrument [Line Items]
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%
Mortgages payable	104,856,000	112,476,000
Notes And Mortgages Payable Variable Rate [Member] | Retail CA Los Angeles Beverly Connection [Member]
Debt Instrument [Line Items]
Mortgages payable	0	100,000,000
Notes And Mortgages Payable Variable Rate [Member] | Other 220 Central Park South [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	10/13
Spread Over LIBOR	2.75%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	2.96%
Mortgages payable	123,750,000	123,750,000
Total Notes And Mortgages Payable [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	4.42%
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	5.70%
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2015 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/15
Interest Rate, End of Period	4.25%
Senior unsecured notes	499,627,000	499,462,000
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2039 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	10/39
Interest Rate, End of Period	7.88%
Senior unsecured notes	460,000,000	460,000,000
Senior Unsecured Notes [Member] | Senior Unsecured Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/22
Interest Rate, End of Period	5.00%
Mortgages payable	398,381,000	398,199,000
Exchangeable Senior Debentures [Member] | Exchangeable Senior Debenture 3.88% [Member]
Debt Instrument [Line Items]
Interest Rate, Stated	3.88%
Convertible Senior Debentures [Member] | Convertible Senior Debenture 2.85% [Member]
Debt Instrument [Line Items]
Interest Rate, Stated	2.85%
Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	1.43%
Unsecured Revolving Credit Facilities [Member] | Unsecured Revolving Credit Facility $1.25 Billion Maturing 11/16 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/16
Spread Over LIBOR	1.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.43%
Revolving credit facility debt	1,150,000,000	138,000,000
Line of Credit Facility, Maximum Borrowing Capacity	1,250,000,000
Unsecured Revolving Credit Facilities [Member] | Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/16 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/16
Spread Over LIBOR	1.35%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Interest Rate, End of Period	1.53%
Revolving credit facility debt	20,000,000	0
Line of Credit Facility, Maximum Borrowing Capacity	1,250,000,000
Unsecured Revolving Credit Facility Reserved For Outstanding Letters Of Credit	22,807,000
Mortgages Payable [Member]
Debt Repayment Of Principal [Line Items]
2013	1,150,439,000
2014	231,117,000
2015	584,802,000
2016	1,585,247,000
2017	1,347,018,000
Thereafter	3,874,900,000
Unsecured Debt [Member]
Debt Repayment Of Principal [Line Items]
2013	0
2014	0
2015	500,000,000
2016	1,170,000,000
2017	0
Thereafter	$ 860,000,000