Commitments And Contingencies (Details) (USD $)	12 Months Ended	0 Months Ended		12 Months Ended
	Dec. 31, 2012 Buildings	Oct. 19, 2012 Pursuit of Transbay Tower [Member] floor sqft	Dec. 31, 2012 Pursuit of Transbay Tower [Member]	Dec. 31, 2012 767 Venture, LLC [Member]	Dec. 31, 2012 John Hancock Tower [Member]	Dec. 31, 2012 601 Lexington Avenue [Member]
Commitments And Contingencies [Line Items]
Letter of credit and performance obligations	$ 13,100,000
Maximum funding obligation				23,700,000
Guarantee Obligation Related To Tenant Re-Leasing Costs					800,000
Maturity Date					Jan. 06, 2017
Property insurance program per occurrence limits	1,000,000,000			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		250,000,000
Coverage for acts of terrorism certified under TRIA				250,000,000
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
Earthquake Coverage Included in Builders Risk Policy for 680 Folsom	20,000,000
Operating partnership guarantee to cover liabilities of IXP	20,000,000
Contractual Tax Agreements Number Of Properties	7
Number of floors in building		61
Area of Real Estate Property		1,400,000
Letters of Credit Outstanding, Amount			5,000,000
Acquisition aggregate purchase price		$ 190,000,000
Companys Joint Venture Ownership Percentage		50.00%
Expiration of Tax Protection for the General Motors Buidling	Jun. 09, 2017