Property Acquisitions (Details) (USD $)	12 Months Ended			1 Months Ended												1 Months Ended			1 Months Ended		1 Months Ended											1 Months Ended			1 Months Ended
	Dec. 31, 2013 sqft	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2013 315 West 33rd Street		Nov. 30, 2013 Retail Properties on Fifth Avenue sqft		Apr. 30, 2013 16 Court Street sqft	Mar. 31, 2013 248-252 Bedford Avenue unit	Dec. 31, 2012 131-137 Spring Street sqft	Dec. 31, 2012 985-987 Third Avenue sqft	Oct. 31, 2012 1080 Amsterdam Avenue sqft unit	Sep. 30, 2012 635-641 Sixth Avenue	Jun. 30, 2012 635-641 Sixth Avenue	Sep. 30, 2013 635-641 Sixth Avenue sqft	Jun. 30, 2012 304 Park Avenue sqft	Oct. 31, 2012 Stonehenge Properties	Jan. 31, 2012 Stonehenge Properties	Jan. 31, 2012 Manhattan	Jan. 31, 2012 Manhattan Series G Preferred Units	Jan. 31, 2012 Retail properties property	Jan. 31, 2012 Multifamily properties property unit 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, 2011 110 East 42nd Street sqft	Apr. 30, 2011 1515 Broadway sqft	Apr. 30, 2011 521 Fifth Avenue	Jan. 31, 2011 521 Fifth Avenue sqft	Mar. 31, 2013 LIBOR 248-252 Bedford Avenue	Nov. 30, 2013 Mixed-Use Residential and Commercial Property 315 West 33rd Street sqft	Nov. 30, 2013 Apartments 315 West 33rd Street unit	Mar. 31, 2013 Apartments 248-252 Bedford Avenue unit	Nov. 30, 2013 Commercial 315 West 33rd Street sqft	Mar. 31, 2013 Townhouses 248-252 Bedford Avenue unit
Property Acquisitions
Area of property (sqft)	28,549,979					66,692		318,000		68,000	42,000	82,250			267,000	215,000						488,000	142,000		1,100,000	205,000	1,750,000		460,000		492,987			270,132
Business Combination, Consideration Transferred						$ 146,000,000		$ 4,000,000		$ 122,300,000	$ 18,000,000		$ 173,000,000			$ 135,000,000			$ 193,100,000				$ 80,000,000
Mortgage encumbering the property								84,600,000												47,600,000
Purchase price fair value adjustment	(2,305,000)	0	498,195,000					(2,300,000)
Number of units (unit)									84													385										333	72		12
Term									5 years												5 years	12 years			5 years			2 years
Face amount of loan									22,000,000.0																344,200,000	16,000,000		150,000,000	140,000,000
Interest rate added to base rate (as a percent)																												2.00%		2.25%
Stated interest rate of preferred partnership units issued (as a percent)									90.00%															6.00%
Recognized Identifiable Assets Acquired and Liabilities Assumed
Land				116,033,000	[1]	43,800,000	[1]	19,217,000	10,865,000	27,021,000	5,400,000			69,848,000		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				270,742,000	[1]	102,200,000	[1]	63,210,000	44,035,000	105,342,000	12,600,000			104,474,000		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				0	[1]	0	[1]	5,122,000	0	179,000	0			0		2,824,000	594,000						5,616,000		7,944,000	823,000	18,298,000		3,318,000
Acquired in-place leases				0	[1]	0	[1]	9,422,000	0	7,046,000	0			7,727,000		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				0	[1]	0	[1]	3,380,000	0	0	0			0		0	2,190,000						0		0	0	27,127,000		0
Assets acquired				386,775,000	[1]	146,000,000	[1]	100,351,000	54,900,000	139,588,000	18,000,000			182,049,000		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										0	0			0		0	0						0		0	0	(3,693,000)		0
Mark-to-market assumed debt				0	[1]	0	[1]	294,000	0
Below market lease value				0	[1]	0	[1]	3,885,000	0	17,288,000	0			9,049,000		5,897,000	13,239,000						7,514,000		20,320,000	2,326,000	84,417,000		25,977,000
Liabilities assumed				0	[1]	0	[1]	4,179,000	0	17,288,000	0			9,049,000		5,897,000	13,239,000						7,514,000		20,320,000	2,326,000	80,724,000		25,977,000
Purchase price allocation				386,775,000	[1]	146,000,000	[1]	96,172,000	54,900,000	122,300,000	18,000,000			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					386,800,000
Net consideration funded by us at closing, excluding consideration financed by debt				386,775,000	[1]	146,000,000	[1]	4,000,000	21,782,000	122,300,000	18,000,000			173,000,000		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				0	[1]	0	[1]	13,835,000	0	0	0			0		0	0						0	2,000,000	0	16,000,000	40,942,000		41,432,000
Debt assumed				0	[1]	0	[1]	84,642,000	0	0	0			0		0	0						0		0	65,000,000	458,767,000		140,000,000
Term of lease												99 years
Number of properties (units)												96
Consideration in cash																102,000,000									41,000,000
Number of properties acquired (properties)																					5	2
Interest rate (as a percent)																				4.50%
Fixed rate debt	4,130,629,000	3,457,769,000																			8,500,000	100,000,000
Interest rate, fixed rate debt (as a percent)																					3.80%	4.10%
Ownership interest in consolidated joint venture (as a percent)																		80.00%
Stake in the joint venture (as a percent)																									49.90%
Value of preferred operating partnership units issued																33,000,000									31,700,000
Consolidated interests in acquiree																									442,300,000		1,200,000,000		245,700,000
Debt assumed																											458,800,000
Purchase price fair value adjustment recognized																									8,300,000		475,100,000		13,800,000
Mortgage on properties assumed																									280,000,000
Senior mezzanine loan									22,000,000.0																344,200,000	16,000,000		150,000,000	140,000,000
Economic Interest (as a percent)																											31.50%
Remaining ownership interest acquired (as a percent)																													49.90%
Cash and other assets																													4,500,000
Fee interest in the property acquired																												$ 15,000,000

[1] We are currently in the process of analyzing the purchase price allocation and, as such, we have not allocated any value to intangible assets such as above- and below-market lease or in-place leases.