Investments in Partially Owned Entities - Debt (Details) (USD $) In Thousands, unless otherwise specified					6 Months Ended				6 Months Ended													6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended		6 Months Ended			6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended				6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Toys R Us [Member]	Dec. 31, 2012 Toys R Us [Member]	Jun. 30, 2013 Toys R Us [Member] Minimum [Member]	Jun. 30, 2013 Toys R Us [Member] Maximum [Member]	Jun. 30, 2013 Alexanders Inc [Member]	Dec. 31, 2012 Alexanders Inc [Member]	Jun. 30, 2013 Alexanders Inc [Member] Minimum [Member]	Jun. 30, 2013 Alexanders Inc [Member] Maximum [Member]	Jun. 30, 2013 Lexington Realty Trust [Member]	Dec. 31, 2012 Lexington Realty Trust [Member]	Jun. 30, 2013 LNR [Member]	Mar. 31, 2013 LNR [Member]	Dec. 31, 2012 LNR [Member]	Jun. 30, 2013 LNR [Member] LNR mortgage notes payable [Member]	Dec. 31, 2012 LNR [Member] LNR mortgage notes payable [Member]	Jun. 30, 2013 LNR [Member] Liabilities Of Consolidated CMBS And CDO Trusts [Member]	Dec. 31, 2012 LNR [Member] Liabilities Of Consolidated CMBS And CDO Trusts [Member]	Jun. 30, 2013 Partially owned office buildings [Member]	Dec. 31, 2012 Partially owned office buildings [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fifth Avenue 666 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Park Avenue 280 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Park Avenue 280 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Washington DC Office Warner Building [Member]	Jun. 30, 2013 Partially owned office buildings [Member] One Park Avenue [Member]	Dec. 31, 2012 Partially owned office buildings [Member] One Park Avenue [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Madison Avenue 330 [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Fairfax Square [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Fairfax Square [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Rosslyn Plaza [Member]	Jun. 30, 2013 Partially owned office buildings [Member] West 57th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] West 57th Street [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Other Mortgage Notes Payable Collateralized By Real Estate [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Other Mortgage Notes Payable Collateralized By Real Estate [Member]	Jun. 30, 2013 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Dec. 31, 2012 Partially owned office buildings [Member] Washington DC Office 1101 17th Street [Member]	Jun. 30, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Dec. 31, 2012 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member]	Jun. 30, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member] Minimum [Member]	Jun. 30, 2013 India real estate ventures [Member] Tcg Urban Infrastructure Holdings [Member] Maximum [Member]	Jun. 30, 2013 Other equity method investments [Member]	Dec. 31, 2012 Other equity method investments [Member]	Jun. 30, 2013 Other equity method investments [Member] Monmouth Mall [Member]	Dec. 31, 2012 Other equity method investments [Member] Monmouth Mall [Member]	Jun. 30, 2013 Other equity method investments [Member] Other Equity Method Investments Other [Member]	Dec. 31, 2012 Other equity method investments [Member] Other Equity Method Investments Other [Member]	Jun. 30, 2013 Other equity method investments [Member] Independence Plaza [Member]	Dec. 31, 2012 Other equity method investments [Member] Independence Plaza [Member]
Schedule Of Long Term Debt Equity Method Investments [Line Items]
Equity method ownership percentage			32.60%				32.40%							26.20%								49.50%		49.50%		55.00%		30.30%		25.00%		20.00%			50.00%				55.00%		25.00%						50.00%				50.10%
Maturity, Range, Start					2014				2014																																		2013
Maturity, Range, End						2021				2018																																		2022
Debt Instrument Maturity Date (String)																						02/19		06/16		05/16		03/16		06/15		12/14			02/14				01/15								09/15				06/18
Debt Instrument, Interest Rate at Period End			7.83%				3.85%															6.76%		6.64%		6.26%		5.00%		1.69%		7.00%		2.69%	4.94%		6.37%		1.44%		13.62%						5.44%		5.00%		3.48%
Equity Method Investment Debt Of Partially Owned Entities			$ 5,158,005	$ 5,683,733			$ 1,058,028	$ 1,065,916			$ 0	$ 1,994,179	$ 0		$ 97,521,521	$ 0	$ 309,787	$ 0	$ 97,211,734	$ 2,781,735	$ 2,731,893	$ 1,139,585	$ 1,109,700	$ 738,462	$ 738,228	$ 292,700	$ 292,700	$ 250,000	$ 250,000	$ 150,000	$ 150,000	$ 69,681	$ 70,127	$ 20,984	$ 19,899	$ 20,434	$ 69,424	$ 69,704	$ 31,000	$ 31,000	$ 222,016	$ 236,579			$ 1,679,400	$ 1,150,543	$ 158,882	$ 159,896	$ 970,518	$ 990,647	$ 550,000	$ 0
Equity Method Investment Pro Rata Share Debt Of Partially Owned Entities	3,831,483	29,443,128
Equity Method Investment Prorata Share Debt Of Partially Owned Entities Excluding Our Pro Rata Share Of Lnr Liabilities Related To Cmbs And Cdo		$ 3,998,929