Property Acquisitions (Details) (USD $)	6 Months Ended		1 Months Ended				1 Months Ended
	Jun. 30, 2012 sqft	Dec. 31, 2011	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 y	Jan. 31, 2012 Multifamily properties property sqft y	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 y	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 y	Jan. 31, 2011 521 Fifth Avenue sqft
Property Acquisitions
Number of properties acquired								5	2
Number of properties with new mortgage								1	2
Area of property (in square feet)	30,776,460		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	3,662,423,000	3,431,353,000							100,000,000
Term (in years)								5	12			5					2
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 (in years)								5	12			5					2
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																	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					65,723,000					44,095,000		191,523,000	34,000,000		462,700,000			110,100,000
Building					122,049,000					33,470,000		233,230,000	46,411,000		707,938,000			146,686,000
Above market lease value					598,000					5,616,000		7,944,000	823,000		18,298,000			3,318,000
Acquired in-place leases					9,038,000					4,333,000		29,948,000	5,396,000		98,661,000			23,016,000
Other assets, net of other liabilities					2,278,000										27,127,000
Assets acquired					199,686,000					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					6,578,000					7,514,000		20,320,000	2,326,000		84,417,000			25,977,000
Liabilities assumed					6,578,000					7,514,000		20,320,000	2,326,000		80,724,000			25,977,000
Purchase price allocation			135,000,000		193,108,000					80,000,000		442,325,000	84,304,000		1,234,000,000	1,230,000,000		257,143,000
Net consideration funded by us at closing			102,000,000		78,121,000					81,632,000		81,835,000	2,744,000		259,228,000			70,000,000
Equity and/or debt investment held	1,014,042,000	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