Property Acquisitions (Details) (USD $)	3 Months Ended							1 Months Ended				1 Months Ended				1 Months Ended
	Sep. 30, 2012 sqft	Dec. 31, 2011	Sep. 30, 2012 635 and 641 Sixth Avenue sqft	Jun. 30, 2012 304 Park Avenue sqft	Oct. 31, 2011 Stonehenge Properties	Jan. 31, 2012 Manhattan	Oct. 31, 2011 Manhattan	Jan. 31, 2012 Manhattan Series G Preferred Units	Jan. 31, 2012 Retail properties property	Jan. 31, 2012 Multifamily properties property sqft	Nov. 30, 2011 51 East 42nd Street sqft	Nov. 30, 2011 51 East 42nd Street Series H Preferred Units	Nov. 30, 2011 180 Maiden Lane sqft	May 31, 2012 110 East 42nd Street	May 31, 2011 110 East 42nd Street sqft	Apr. 30, 2012 1515 Broadway	Apr. 30, 2011 1515 Broadway sqft	Apr. 30, 2011 521 Fifth Avenue	Jan. 31, 2011 521 Fifth Avenue sqft
Property Acquisitions
Number of properties acquired									5	2
Number of properties with new mortgage									1	2
Stated interest rate of mortgage loan (as a percent)	9.30%
Area of property (in square feet)	29,537,460		267,000	215,000						488,000	142,000		1,100,000		205,000		1,750,000		460,000
Stated interest rate of preferred partnership units issued (as a percent)								4.50%				6.00%
Value of preferred operating partnership units issued				$ 33,000,000				$ 47,600,000				$ 2,000,000	$ 31,700,000
Number of units										385
Fixed rate debt	3,654,167,000	3,431,353,000								100,000,000
Term									5 years	12 years			5 years					2 years
Interest rate, fixed rate debt (as a percent)										4.125%
Ownership interest in consolidated joint venture (as a percent)					80.00%		80.00%
Stake in the joint venture (as a percent)													49.90%
Consideration in cash				102,000,000									41,000,000
Refinanced mortgage loan													344,200,000		16,000,000			150,000,000
Economic Interest (as a percent)																	31.50%
Term of refinanced mortgage									5 years	12 years			5 years					2 years
Mortgage on properties assumed									8,500,000				280,000,000
Senior mezzanine loan													344,200,000		16,000,000			150,000,000
Purchase price fair value adjustment recognized													8,300,000				475,100,000		13,800,000
Remaining ownership interest acquired (as a percent)																			49.90%
Consolidated interests in acquiree																	1,230,000,000		245,700,000
Interest rate, description																90-Day LIBOR		30-day LIBOR
Interest rate added to base rate (as a percent)																		2.00%
Fee interest in the property acquired																		15,000,000
Allocation of the purchase price of the assets acquired and liabilities assumed
Land				54,189,000		65,533,000					44,095,000		191,523,000	34,000,000			462,700,000		110,100,000
Building and building leasehold				75,619,000		128,457,000					33,470,000		233,230,000	46,411,000			707,938,000		146,686,000
Above market lease value				2,824,000		594,000					5,616,000		7,944,000	823,000			18,298,000		3,318,000
Acquired in-place leases				8,265,000		9,573,000					4,333,000		29,948,000	5,396,000			98,661,000		23,016,000
Other assets, net of other liabilities						2,190,000											27,127,000
Assets acquired				140,897,000		206,347,000					87,514,000		462,645,000	86,630,000			1,314,724,000		283,120,000
Fair value adjustment to mortgage note payable																	(3,693,000)
Below market lease value				5,897,000		13,239,000					7,514,000		20,320,000	2,326,000			84,417,000		25,977,000
Liabilities assumed				5,897,000		13,239,000					7,514,000		20,320,000	2,326,000			80,724,000		25,977,000
Purchase price allocation			173,000,000	135,000,000		193,108,000					80,000,000		442,325,000	84,304,000			1,234,000,000		257,143,000
Net consideration funded by us at closing				135,000,000		78,121,000					81,632,000		81,835,000	2,744,000			259,228,000		70,000,000
Equity and/or debt investment held	1,020,790,000	893,933,000												16,000,000			40,942,000		41,432,000
Debt assumed														65,000,000			458,767,000		140,000,000
Net assets acquired, including previous investments in unconsolidated joint ventures																	1,234,000,000
Cash and other assets acquired																			$ 4,500,000