Property Acquisitions (Details) (USD $)					1 Months Ended					0 Months Ended		1 Months Ended	0 Months Ended				0 Months Ended		1 Months Ended	0 Months Ended	1 Months Ended
	Dec. 31, 2012 sqft	Dec. 31, 2011	Dec. 31, 2012 131-137 Spring Street sqft	Dec. 31, 2012 985-987 Third Avenue sqft	Oct. 31, 2012 1080 Amsterdam Avenue item sqft	Dec. 31, 2012 635 and 641 Sixth Avenue	Sep. 30, 2012 635 and 641 Sixth Avenue sqft	Dec. 31, 2012 304 Park Avenue South	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 Retail properties	Jan. 31, 2012 Multifamily properties item sqft	Dec. 31, 2012 Stonehenge Properties	Jan. 31, 2012 Stonehenge Properties	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 item sqft	Aug. 31, 2010 125 Park Avenue	Dec. 31, 2010 Gramercy Capital Corp.	Dec. 31, 2010 885 Third Avenue	Dec. 31, 2010 2 Herald Square	Dec. 31, 2010 292 Madison Avenue	Dec. 31, 2010 Williamsburg, Brooklyn item
Property Acquisitions
Area of property (in square feet)	29,685,079		68,000	42,000	82,250		267,000		215,000				488,000			142,000		1,100,000	205,000	1,750,000		460,000	1,050,000
Purchase consideration			$ 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	$ 15,000,000	$ 183,750,000	$ 86,864,000				$ 18,385,000
Term of lease					99 years
Consideration in cash									102,000,000									41,000,000
Value of operating partnership units issued									33,000,000	47,600,000							2,000,000	31,700,000
Number of properties acquired											5		2																2
Stated interest rate of preferred partnership units issued (as a percent)										4.50%							6.00%
Number of units					96								385
Term												5 years	12 years					5 years			2 years
Fixed rate debt	3,457,769,000	3,431,353,000									8,500,000	8,500,000	100,000,000
Interest rate, fixed rate debt (as a percent)											3.75%	3.75%	4.125%
Number of properties with new mortgage											1
Stake in the joint venture (as a percent)															80.00%			49.90%
Purchase price fair value adjustment recognized																		8,300,000		475,100,000		13,800,000
Refinanced mortgage loan																		344,200,000	16,000,000		150,000,000
Mortgage on properties assumed																		280,000,000	65,000,000	458,767,000		140,000,000	139,672,000	146,250,000	265,604,000	120,400,000	86,100,000	59,100,000
Economic interest purchased (as a percent)																				31.50%
Consolidated interest in acquiree																						245,700,000
Remaining ownership interest acquired (as a percent)																						49.90%				45.00%	45.00%
Cash and other assets acquired																						4,500,000
Interest rate added to base rate (as a percent)																					2.00%		3.50%
Interest rate, description																					30-day LIBOR		30-day LIBOR
Fee interest in the property acquired																					15,000,000
Interest in acquired loan (as a percent)																							50.00%
Number of loans acquired																							2
Additional capital funded																							15,000,000
Amount of mortgage restructured																							139,700,000
Interest rate (as a percent)																								5.748%
Purchase price of property, excluding mortgage debt assumed																										39,300,000	25,600,000	19,200,000
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	501,021,000				6,200,000
Building and building leasehold						104,474,000		75,619,000						128,457,000
Building																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	23,178,000				2,304,000
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	741,511,000				18,662,000
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	24,897,000				277,000
Purchase price allocation						173,000,000	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	180,834,000	330,000,000	716,614,000				18,385,000
Net consideration funded by us at closing			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	15,000,000	183,750,000	86,864,000				18,385,000
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																		$ 280,000,000	$ 65,000,000	$ 458,767,000		$ 140,000,000	$ 139,672,000	$ 146,250,000	$ 265,604,000	$ 120,400,000	$ 86,100,000	$ 59,100,000