Property Acquisitions (Details) (USD $)						1 Months Ended			0 Months Ended				1 Months Ended
	Mar. 31, 2013 sqft	Dec. 31, 2012	Apr. 30, 2012	Dec. 31, 2012 131-137 Spring Street sqft	Dec. 31, 2012 985-987 Third Avenue sqft	Oct. 31, 2012 1080 Amsterdam Avenue item sqft	Sep. 30, 2012 635 and 641 Sixth Avenue sqft	Jun. 30, 2012 304 Park Avenue South sqft	Jan. 31, 2012 Manhattan Series G preferred units	Jan. 31, 2012 Retail properties item	Jan. 31, 2012 Multifamily properties item sqft	Jan. 31, 2012 Stonehenge Properties	Mar. 31, 2013 248-252 Bedford Avenue item
Property Acquisitions
Floating interest rate mortgage loan	$ 466,079,000	$ 1,157,695,000											$ 22,000,000
Interest rate, basis points (as a percent)													2.25%
Interest rate, description													LIBOR
Number of apartment units acquired													72
Number of townhouses acquired													12
Number of units						96					385
Purchase consideration							173,000,000	135,000,000				78,121,000	54,900,000
Area of property (in square feet)	29,721,879			68,000	42,000	82,250	267,000	215,000			488,000
Term of lease						99 years
Consideration in cash								102,000,000
Value of operating partnership units issued								33,000,000	47,600,000
Number of properties acquired										5	2		84
Stated interest rate of preferred partnership units issued (as a percent)									4.50%
Term										5 years	12 years		5 years
Fixed rate debt	4,349,406,000	3,457,769,000								8,500,000	100,000,000
Interest rate, fixed rate debt (as a percent)										3.75%	4.125%
Number of properties with new mortgage										1
Stake in the joint venture (as a percent)			32.28%
Ownership interest in consolidated joint venture (as a percent)												80.00%	90.00%
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				122,300,000	18,000,000		173,000,000	135,000,000				193,108,000
Net consideration funded by us at closing							$ 173,000,000	$ 135,000,000				$ 78,121,000	$ 54,900,000