Commitments And Contingencies (Narrative) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012
Commitments And Contingencies Disclosure [Line Items]
Letter of credit and performance obligations	$ 12,900,000
Property insurance program per occurrence limits	1,000,000,000
Per occurrence limit for NBCR Coverage	1,000,000,000
Value of program trigger	100,000,000
Coinsurance of program trigger	15.00%
Deductible in insurance as a percentage of the value of the affected property, San Francisco	5.00%
Per occurrence limit of the earthquake insurance which covers San Francisco region	120,000,000
Annual aggregate limit of the earthquake insurance which covers San Francisco region	120,000,000
Amount of earthquake insurance provided by IXP, LLC as direct insurer San Francisco	20,000,000
Annual aggregate limit of the earthquake insurance which covers San Francisco region	20,000,000
Operating partnership guarantee to cover liabilities of IXP	20,000,000
601 Lexington Avenue [Member]
Commitments And Contingencies Disclosure [Line Items]
Coverage for acts of Terrorism certified under TRIA in excess of amount provided by NYXP, LLC, as a direct insurer	250,000,000
767 Venture, LLC [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum funding obligation	9,800,000
Property insurance program per occurrence limits	1,625,000,000
Coverage for acts of Terrorism certified under TRIA in excess of amount provided by NYXP, LLC, as a direct insurer	1,375,000,000
Terrorism Coverage	250,000,000
John Hancock Tower Property [Member]
Commitments And Contingencies Disclosure [Line Items]
Maximum funding obligation	$ 800,000
Mortgage financing maturity date	Jan. 06, 2017