Property Acquisitions (Details) (USD $)			1 Months Ended							1 Months Ended		1 Months Ended
	Mar. 31, 2013 sqft property	Dec. 31, 2012	Mar. 31, 2013 248-252 Bedford Avenue property unit	Mar. 31, 2013 248-252 Bedford Avenue Mortgage loan	Sep. 30, 2012 635 and 641 Sixth Avenue sqft	Jun. 30, 2012 304 Park Avenue sqft	Oct. 31, 2011 Stonehenge Properties	Dec. 31, 2012 131-137 Spring Street sqft	Dec. 31, 2012 985-987 Third Avenue sqft	Oct. 31, 2012 1080 Amsterdam Avenue sqft property	Jan. 31, 2012 Manhattan	Jan. 31, 2012 Manhattan Series G Preferred Units	Jan. 31, 2012 Retail properties property	Jan. 31, 2012 Multifamily properties property sqft
Property Acquisitions
Number of properties acquired			84										5	2
Number of apartment units acquired			72
Number of townhouses acquired			12
Extinguishment of debt				$ 22,000,000
Interest rate added to base rate (as a percent)				2.25%
Interest rate, description				LIBOR
Area of property (in square foot)	29,721,879				267,000	215,000		68,000	42,000	82,250				488,000
Term of lease										99 years
Number of Properties	67									96
Consideration in cash						102,000,000
Value of preferred operating partnership units issued						33,000,000						47,600,000
Stated interest rate of preferred partnership units issued (as a percent)												4.50%
Number of units														385
Fixed rate debt	4,349,406,000	3,457,769,000											8,500,000	100,000,000
Term				5 years									5 years	12 years
Interest rate, fixed rate debt (as a percent)													3.75%	4.125%
Ownership interest in consolidated joint venture (as a percent)			90.00%				80.00%
Allocation of the purchase price of the assets acquired and liabilities assumed
Land					69,848,000	54,189,000					65,533,000
Building and building leasehold					104,474,000	75,619,000					128,457,000
Above market lease value						2,824,000					594,000
Acquired in-place leases					7,727,000	8,265,000					9,573,000
Other assets, net of other liabilities											2,190,000
Assets acquired					182,049,000	140,897,000					206,347,000
Below market lease value					9,049,000	5,897,000					13,239,000
Liabilities assumed					9,049,000	5,897,000					13,239,000
Purchase price allocation					173,000,000	135,000,000		122,300,000	18,000,000		193,108,000
Net consideration funded by us at closing			$ 54,900,000		$ 173,000,000	$ 135,000,000					$ 78,121,000