Investments in Partially Owned Entities - Debt (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity Method Investment Pro Rata Share Debt Of Partially Owned Entities	$ 25,648,473	$ 37,531,298
Equity Method Investment Prorata Share Debt Of Partially Owned Entities Excluding Our Pro Rata Share Of Liabilities Related To Cmbs And Cdo	4,049,108	4,199,145
Toys R Us [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	32.50%	32.70%
Debt Instrument, Maturity Date Range, Start	2013
Debt Instrument, Maturity Date Range, End	2021
Debt Instrument, Interest Rate at Period End	7.40%
Equity Method Investment Debt Of Partially Owned Entities	5,423,735	6,047,521
Alexander's Inc. [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	32.40%
Debt Instrument, Maturity Date Range, Start	2013
Debt Instrument, Maturity Date Range, End	2018
Debt Instrument, Interest Rate at Period End	3.50%
Equity Method Investment Debt Of Partially Owned Entities	1,319,776	1,330,932
Lexington Realty Trust [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	11.80%	12.00%
Debt Instrument, Maturity Date Range, Start	2012
Debt Instrument, Maturity Date Range, End	2037
Debt Instrument, Interest Rate at Period End	5.45%
Equity Method Investment Debt Of Partially Owned Entities	1,739,466	1,712,750
LNR [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	26.20%
Equity Method Investment Debt Of Partially Owned Entities	82,989,102	127,701,840
LNR [Member] | Liabilities Of Consolidated Cmbs And Cdo Trusts [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Debt Instrument, Interest Rate at Period End	5.32%
Equity Method Investment Debt Of Partially Owned Entities	82,522,220	127,348,336
LNR [Member] | Mortgage notes payable [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Debt Instrument, Maturity Date Range, Start	2013
Debt Instrument, Maturity Date Range, End	2031
Debt Instrument, Interest Rate at Period End	3.89%
Equity Method Investment Debt Of Partially Owned Entities	466,882	353,504
Partially owned office buildings [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity Method Investment Debt Of Partially Owned Entities	2,682,112	2,686,010
Partially owned office buildings [Member] | Fifth Avenue 666 [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	49.50%
Debt Instrument Maturity Date (String)	02/19
Debt Instrument, Interest Rate at Period End	6.76%
Equity Method Investment Debt Of Partially Owned Entities	1,090,592	1,035,884
Partially owned office buildings [Member] | 280 Park Avenue [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	49.50%
Debt Instrument Maturity Date (String)	06/16
Debt Instrument, Interest Rate at Period End	6.65%
Equity Method Investment Debt Of Partially Owned Entities	738,009	737,678
Partially owned office buildings [Member] | Warner Building [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	55.00%
Debt Instrument Maturity Date (String)	05/16
Debt Instrument, Interest Rate at Period End	6.26%
Equity Method Investment Debt Of Partially Owned Entities	292,700	292,700
Partially owned office buildings [Member] | One Park Avenue [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	30.30%
Debt Instrument Maturity Date (String)	03/16
Debt Instrument, Interest Rate at Period End	5.00%
Equity Method Investment Debt Of Partially Owned Entities	250,000	250,000
Partially owned office buildings [Member] | 330 Madison Avenue [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	25.00%
Debt Instrument Maturity Date (String)	06/15
Debt Instrument, Interest Rate at Period End	1.73%
Equity Method Investment Debt Of Partially Owned Entities	150,000	150,000
Partially owned office buildings [Member] | Fairfax Square [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	20.00%
Debt Instrument Maturity Date (String)	12/14
Debt Instrument, Interest Rate at Period End	7.00%
Equity Method Investment Debt Of Partially Owned Entities	70,344	70,974
Partially owned office buildings [Member] | Rosslyn Plaza [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity Method Investment Debt Of Partially Owned Entities	0	56,680
Partially owned office buildings [Member] | Rosslyn Plaza [Member] | Range Minimum
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	43.70%
Partially owned office buildings [Member] | Rosslyn Plaza [Member] | Range Maximum
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	50.40%
Partially owned office buildings [Member] | West 57th Street [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	50.00%
Debt Instrument Maturity Date (String)	02/14
Debt Instrument, Interest Rate at Period End	4.94%
Equity Method Investment Debt Of Partially Owned Entities	20,628	21,864
Partially owned office buildings [Member] | Other partially owned office buildings [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Debt Instrument, Interest Rate at Period End	6.38%
Equity Method Investment Debt Of Partially Owned Entities	69,839	70,230
India real estate ventures [Member] | Range Minimum
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	4.00%
India real estate ventures [Member] | Range Maximum
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	36.50%
India real estate ventures [Member] | Tcg Urban Infrastructure Holdings [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	25.00%
Debt Instrument, Maturity Date Range, Start	2012
Debt Instrument, Maturity Date Range, End	2022
Debt Instrument, Interest Rate at Period End	13.13%
Equity Method Investment Debt Of Partially Owned Entities	241,208	226,534
Other equity method investments [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity Method Investment Debt Of Partially Owned Entities	1,657,882	1,495,403
Other equity method investments [Member] | Verde Realty Operating Partnership [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	8.30%
Debt Instrument, Maturity Date Range, Start	2013
Debt Instrument, Maturity Date Range, End	2025
Debt Instrument, Interest Rate at Period End	5.52%
Equity Method Investment Debt Of Partially Owned Entities	503,211	340,378
Other equity method investments [Member] | Monmouth Mall [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage	50.00%
Debt Instrument Maturity Date (String)	09/15
Debt Instrument, Interest Rate at Period End	5.44%
Equity Method Investment Debt Of Partially Owned Entities	160,662	162,153
Other equity method investments [Member] | Other Equity Method Investments Other [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Debt Instrument, Interest Rate at Period End	4.93%
Equity Method Investment Debt Of Partially Owned Entities	$ 994,009	$ 992,872