Debt (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Mar. 28, 2013
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	$ 8,566,582,000	$ 8,644,200,000
Senior unsecured notes (in US Dollars)	1,350,769,000	1,358,008,000
Revolving credit facility debt (in US Dollars)	83,982,000	1,170,000,000
Outstanding letters of credit	33,068,000
Notes And Mortgages Payable Fixed Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	4.91%
Mortgages payable (in US Dollars)	7,297,467,000	6,771,001,000
Notes And Mortgages Payable Fixed Rate [Member] | New York 1290 Avenue Of Americas [Member]
Debt Instrument [Line Items]
Equity method ownership percentage	70.00%
Debt Instrument Maturity	11/22
Interest Rate, End of Period	3.34%
Mortgages payable (in US Dollars)	950,000,000	950,000,000
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 (in US Dollars)	425,000,000	425,000,000
Notes And Mortgages Payable Fixed Rate [Member] | New York Office 666 Fifth Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	03/23
Interest Rate, End of Period	3.61%
Mortgages payable (in US Dollars)	390,000,000	0
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 (in US Dollars)	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 (in US Dollars)	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 (in US Dollars)	300,000,000	300,000,000
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 (in US Dollars)	196,013,000	199,198,000
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 (in US Dollars)	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 (in US Dollars)	30,872,000	31,253,000
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 (in US Dollars)	736,259,000	704,957,000
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 (in US Dollars)	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 (in US Dollars)	150,000,000	150,000,000
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 (in US Dollars)	149,005,000	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 (in US Dollars)	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 (in US Dollars)	103,794,000	105,724,000
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 (in US Dollars)	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	6.56%
Mortgages payable (in US Dollars)	89,321,000	93,226,000
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 (in US Dollars)	78,875,000	79,624,000
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 (in US Dollars)	71,841,000	74,053,000
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 (in US Dollars)	73,003,000	73,939,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 2231 Crystal Drive [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	41,298,000
Notes And Mortgages Payable Fixed Rate [Member] | Washington DC 1225 Clark Street [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	24,834,000
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	4.25%
Mortgages payable (in US Dollars)	563,692,000	573,180,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Bergen Town Center [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/23
Interest Rate, End of Period	3.56%
Mortgages payable (in US Dollars)	300,000,000	0
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 (in US Dollars)	120,000,000	120,000,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Broadway Mall [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	85,180,000
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 (in US Dollars)	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 (in US Dollars)	52,822,000	54,101,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	68,659,000	86,641,000
Maturity, Range, Start	06/14
Maturity, Range, End	11/34
Interest Rate, Range, Minimum	5.12%
Interest Rate, Range, Maximum	7.30%
Notes And Mortgages Payable Fixed Rate [Member] | Other 555 California Street [Member]
Debt Instrument [Line Items]
Equity method ownership percentage	70.00%
Debt Instrument Maturity	09/21
Interest Rate, End of Period	5.10%
Mortgages payable (in US Dollars)	600,000,000	600,000,000
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 (in US Dollars)	550,000,000	550,000,000
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 (in US Dollars)	59,518,000	60,000,000
Notes And Mortgages Payable Variable Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	2.39%
Mortgages payable (in US Dollars)	1,269,115,000	1,873,199,000
Notes And Mortgages Payable Variable Rate [Member] | New York Independence Plaza [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	334,225,000
Notes And Mortgages Payable Variable Rate [Member] | New York Eleven Penn Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/19
Interest Rate, End of Period	2.53%
Mortgages payable (in US Dollars)	330,000,000	330,000,000
Spread Over LIBOR	2.35%
Debt Instrument, Description of Variable Rate Basis	LIBOR
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
Interest Rate, End of Period	2.68%
Mortgages payable (in US Dollars)	325,000,000	325,000,000
Spread Over LIBOR	2.50%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | New York Office 4 Union Square South [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	11/19
Interest Rate, End of Period	2.33%
Mortgages payable (in US Dollars)	120,000,000	120,000,000
Spread Over LIBOR	2.15%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | New York 435 Seventh Avenue [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	08/19
Interest Rate, End of Period	2.43%
Mortgages payable (in US Dollars)	98,000,000	98,000,000
Spread Over LIBOR	2.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | New York 866 UN Plaza [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	05/16
Interest Rate, End of Period	1.43%
Mortgages payable (in US Dollars)	44,978,000	44,978,000
Spread Over LIBOR		1.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | Washington DC River House Apartments [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	04/18
Interest Rate, End of Period	1.58%
Mortgages payable (in US Dollars)	64,000,000	64,000,000
Spread Over LIBOR	1.53%
Debt Instrument, Description of Variable Rate Basis	Freddie Mac
Notes And Mortgages Payable Variable Rate [Member] | Washington DC 2200/2300 Clarendon Boulevard [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	01/15
Interest Rate, End of Period	0.93%
Mortgages payable (in US Dollars)	42,806,000	47,353,000
Spread Over LIBOR	0.75%
Debt Instrument, Description of Variable Rate Basis	LIBOR
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
Interest Rate, End of Period	1.58%
Mortgages payable (in US Dollars)	43,581,000	43,581,000
Spread Over LIBOR	1.40%
Debt Instrument, Description of Variable Rate Basis	LIBOR
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
Interest Rate, End of Period	2.36%
Mortgages payable (in US Dollars)	60,000,000	60,000,000
Spread Over LIBOR	1.36%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | Retail Bergen Town Center [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	282,312,000
Notes And Mortgages Payable Variable Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	07/18
Interest Rate, End of Period	1.48%
Mortgages payable (in US Dollars)	17,000,000	0
Spread Over LIBOR	1.30%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Notes And Mortgages Payable Variable Rate [Member] | Other 220 Central Park South [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	10/13
Interest Rate, End of Period	2.93%
Mortgages payable (in US Dollars)	123,750,000	123,750,000
Spread Over LIBOR	2.75%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Total Notes And Mortgages Payable [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	4.54%
Mortgages payable (in US Dollars)	8,566,582,000	8,644,200,000
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	5.69%
Senior unsecured notes (in US Dollars)	1,350,769,000	1,358,008,000
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 (in US Dollars)	499,752,000	499,627,000
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 (in US Dollars)	452,500,000	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%
Senior unsecured notes (in US Dollars)	398,517,000	398,381,000
Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	1.33%
Revolving credit facility debt (in US Dollars)	83,982,000	1,170,000,000
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
Revolving credit facility debt (in US Dollars)	0	1,150,000,000
Line of Credit Facility, Maximum Borrowing Capacity	1,250,000,000
Unsecured Revolving Credit Facilities [Member] | Unsecured Revolving Credit Facility $1.25 Billion Maturing 06/18 [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	06/18
Interest Rate, End of Period	1.33%
Spread Over LIBOR	1.15%		1.35%
Debt Instrument, Description of Variable Rate Basis	LIBOR	LIBOR
Revolving credit facility debt (in US Dollars)	83,982,000	20,000,000
Line of Credit Facility, Maximum Borrowing Capacity	1,250,000,000
Outstanding letters of credit	$ 33,068,000	$ 22,807,000