Property Acquisitions (Details) (USD $)								1 Months Ended		1 Months Ended				1 Months Ended							12 Months Ended	1 Months Ended		12 Months Ended	1 Months Ended
	Dec. 31, 2012 property 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	Dec. 31, 2012 131-137 Spring Street sqft	Dec. 31, 2012 985-987 Third Avenue sqft	Oct. 31, 2012 1080 Amsterdam Avenue property sqft	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	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	Jan. 31, 2010 100 Church Street sqft	Dec. 31, 2007 100 Church Street item	Aug. 30, 2010 125 Park Avenue	Dec. 31, 2010 Williamsburg, Brooklyn property	Dec. 31, 2010 Gramercy Capital Corp.	Dec. 31, 2010 885 Third Avenue	Dec. 31, 2010 2 Herald Square	Dec. 31, 2010 292 Madison Avenue
Property Acquisitions
Number of properties acquired											5	2
Area of property (in square foot)	29,685,079		267,000	215,000		68,000	42,000	82,250				488,000	142,000		1,100,000	205,000	1,750,000		460,000	1,050,000
Term of lease								99 years
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	3,457,769,000	3,431,353,000									8,500,000	100,000,000
Term											5 years	12 years			5 years
Interest rate, fixed rate debt (as a percent)											3.75%	4.125%
Ownership interest in consolidated joint venture (as a percent)					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		139,700,000
Economic Interest (as a percent)																	31.50%
Term of refinanced mortgage											5 years	12 years			5 years
Mortgage on properties assumed															280,000,000						40,900,000
Interest in acquired loan (as a percent)																					50.00%
Number of loans acquired																					2
Senior mezzanine loan															344,200,000	16,000,000		150,000,000		139,700,000
Purchase price fair value adjustment recognized															8,300,000		475,100,000		13,800,000
Remaining ownership interest acquired (as a percent)																			49.90%						45.00%	45.00%
Purchase price of property, excluding mortgage debt assumed																									39,300,000	25,600,000	19,200,000
Consolidated interests in acquiree															442,300,000		1,230,000,000		245,700,000
Mortgage on properties assumed																						146,250,000			120,400,000	86,100,000	59,100,000
Number of Properties	66							96															2
Interest rate, description																		30-day LIBOR		30-day LIBOR
Interest rate added to base rate (as a percent)																		2.20%		3.50%
Fee interest in the property acquired																		15,000,000
Allocation of the purchase price of the assets acquired and liabilities assumed
Land			69,848,000	54,189,000					65,533,000				44,095,000		191,523,000	34,000,000	462,700,000		110,100,000	32,494,000		120,900,000	6,200,000	501,021,000
Building and building leasehold			104,474,000	75,619,000					128,457,000				33,470,000		233,230,000	46,411,000	707,938,000		146,686,000	86,806,000		201,726,000	10,158,000
Above market lease value				2,824,000					594,000				5,616,000		7,944,000	823,000	18,298,000		3,318,000	118,000		11,282,000	2,304,000	23,178,000
Acquired in-place leases			7,727,000	8,265,000					9,573,000				4,333,000		29,948,000	5,396,000	98,661,000		23,016,000	17,380,000		28,828,000		217,312,000
Restricted cash																				53,735,000
Other assets, net of other liabilities									2,190,000								27,127,000
Assets acquired			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	190,533,000		362,736,000	18,662,000	741,511,000
Fair value adjustment to mortgage note payable																	(3,693,000)					12,147,000		22,806,000
Below market lease value			9,049,000	5,897,000					13,239,000				7,514,000		20,320,000	2,326,000	84,417,000		25,977,000	8,025,000		20,589,000	277,000
Other liabilities, net of other assets																				1,674,000				2,091,000
Liabilities assumed			9,049,000	5,897,000					13,239,000				7,514,000		20,320,000	2,326,000	80,724,000		25,977,000	9,699,000		32,736,000	277,000	24,897,000
Purchase price allocation			173,000,000	135,000,000		122,300,000	18,000,000		193,108,000				80,000,000		442,325,000	84,304,000	1,234,000,000		257,143,000	180,834,000		330,000,000	18,385,000	716,614,000
Net consideration funded by us at closing			173,000,000	135,000,000					78,121,000				81,632,000		81,835,000	2,744,000	259,228,000		70,000,000	15,000,000		183,750,000	18,385,000	86,864,000
Equity and/or debt investment held	1,032,243,000	893,933,000														16,000,000	40,942,000		41,432,000	40,938,000				111,751,000
Debt assumed																65,000,000	458,767,000		140,000,000	139,672,000		146,250,000		265,604,000
Net assets acquired, including previous investments in unconsolidated joint ventures																						330,000,000
Interest rate (as a percent)																						5.748%
Cash and other assets acquired																			$ 4,500,000