Debt (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	$ 8,582,573	$ 8,663,326
Senior unsecured notes (in US Dollars)	1,358,182	1,358,008
Revolving credit facility debt (in US Dollars)	83,982	1,170,000
Notes And Mortgages Payable Fixed Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	4.91%
Mortgages payable (in US Dollars)	7,312,813	6,771,001
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 (in US Dollars)	950,000	950,000
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	425,000
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	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	353,000
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	318,554
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	300,000
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)	197,069	199,198
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	80,000
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)	30,998	31,253
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)	725,559	704,957
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	195,546
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	150,000
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)	149,506	150,000
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	115,022
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)	104,522	105,724
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	101,671
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	6.54%
Mortgages payable (in US Dollars)	90,633	93,226
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)	79,129	79,624
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)	72,592	74,053
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)	73,312	73,939
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
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
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)	566,886	573,180
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	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	120,000
Notes And Mortgages Payable Fixed Rate [Member] | Retail Broadway Mall [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	85,180
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	75,000
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)	53,308	54,101
Notes And Mortgages Payable Fixed Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	85,789	86,641
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 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 (in US Dollars)	600,000	600,000
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	550,000
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)	59,717	60,000
Notes And Mortgages Payable Variable Rate [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	2.42%
Mortgages payable (in US Dollars)	1,269,760	1,892,325
Notes And Mortgages Payable Variable Rate [Member] | New York Independence Plaza [Member]
Debt Instrument [Line Items]
Mortgages payable (in US Dollars)	0	334,225
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	2.54%
Mortgages payable (in US Dollars)	330,000	330,000
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	2.69%
Mortgages payable (in US Dollars)	325,000	325,000
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	2.34%
Mortgages payable (in US Dollars)	120,000	120,000
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	2.44%
Mortgages payable (in US Dollars)	98,000	98,000
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	1.44%
Mortgages payable (in US Dollars)	44,978	44,978
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	1.55%
Mortgages payable (in US Dollars)	64,000	64,000
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	0.94%
Mortgages payable (in US Dollars)	44,325	47,353
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	1.59%
Mortgages payable (in US Dollars)	43,581	43,581
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	60,000
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
Notes And Mortgages Payable Variable Rate [Member] | Retail Other [Member]
Debt Instrument [Line Items]
Debt Instrument Maturity	05/15
Interest Rate, End of Period	3.45%
Mortgages payable (in US Dollars)	16,126	19,126
Spread Over LIBOR	3.25%
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	2.94%
Mortgages payable (in US Dollars)	123,750	123,750
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	4.55%
Mortgages payable (in US Dollars)	8,582,573	8,663,326
Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	5.70%
Senior unsecured notes (in US Dollars)	1,358,182	1,358,008
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)	499,710	499,627
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)	460,000	460,000
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)	398,472	398,381
Unsecured Revolving Credit Facilities [Member]
Debt Instrument [Line Items]
Interest Rate, End of Period	1.32%
Revolving credit facility debt (in US Dollars)	83,982	1,170,000
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
Interest Rate, End of Period
Spread Over LIBOR	1.25%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Revolving credit facility debt (in US Dollars)	0	1,150,000
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	1.32%
Spread Over LIBOR	1.15%	1.35%
Debt Instrument, Description of Variable Rate Basis	LIBOR
Revolving credit facility debt (in US Dollars)	$ 83,982	$ 20,000