Investment in Unconsolidated Joint Ventures (Details 2) (USD $)			1 Months Ended				1 Months Ended	3 Months Ended			12 Months Ended																									12 Months Ended									1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended											1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Nov. 30, 2012 521 Fifth Avenue	Apr. 30, 2011 521 Fifth Avenue	Jan. 31, 2011 521 Fifth Avenue	Dec. 31, 2011 521 Fifth Avenue	Sep. 30, 2012 West Coast office portfolio Mezzanine loans	Sep. 30, 2012 West Coast office portfolio Mezzanine loans	Dec. 31, 2012 717 Fifth Avenue Mortgage loan	Dec. 31, 2012 717 Fifth Avenue Mezzanine loans	Dec. 31, 2012 Joint venture	Dec. 31, 2011 Joint venture	Dec. 31, 2010 Joint venture	Dec. 31, 2012 Joint venture 388 and 390 Greenwich Street	Dec. 31, 2011 Joint venture 388 and 390 Greenwich Street	Dec. 31, 2012 Joint venture 388 and 390 Greenwich Street Mortgage loan	Dec. 31, 2012 Joint venture 388 and 390 Greenwich Street Mezzanine loans	Dec. 31, 2012 Joint venture 315 West 36th Street	Dec. 31, 2012 Joint venture 800 Third Avenue	Dec. 31, 2011 Joint venture 800 Third Avenue	Dec. 31, 2012 Joint venture 1 and 2 Jericho Plaza	Dec. 31, 2011 Joint venture 1 and 2 Jericho Plaza	Dec. 31, 2012 Joint venture 1745 Broadway	Dec. 31, 2011 Joint venture 1745 Broadway	Dec. 31, 2012 Joint venture 21 West 34th Street	Dec. 31, 2011 Joint venture 21 West 34th Street	Dec. 31, 2012 Joint venture 280 Park Avenue	Dec. 31, 2011 Joint venture 280 Park Avenue	Dec. 31, 2012 Joint venture 11 West 34th Street	Dec. 31, 2011 Joint venture 11 West 34th Street	Dec. 31, 2011 Joint venture One Court Square	Dec. 31, 2012 Joint venture 7 Renaissance	Dec. 31, 2012 Joint venture 100 Park Avenue	Dec. 31, 2011 Joint venture 100 Park Avenue	Dec. 31, 2012 Joint venture 21 East 66th Street	Dec. 31, 2012 Joint venture 521 Fifth Avenue	Dec. 31, 2012 Joint venture 1604-1610 Broadway	Dec. 31, 2011 Joint venture 1604-1610 Broadway	Dec. 31, 2011 Joint venture 141 Fifth Avenue	Dec. 31, 2012 Joint venture 600 Lexington Avenue	Dec. 31, 2011 Joint venture 600 Lexington Avenue	Dec. 31, 2012 Joint venture 33 Beekman	Dec. 31, 2012 Joint venture 10 East 53rd Street	Dec. 31, 2012 Joint venture 724 Fifth Avenue	Apr. 30, 2011 Joint venture 3 Columbus Circle	Dec. 31, 2012 Joint venture 3 Columbus Circle	Dec. 31, 2011 Joint venture 3 Columbus Circle	Aug. 31, 2012 Joint venture The Meadows	Dec. 31, 2012 Joint venture The Meadows	Dec. 31, 2011 Joint venture The Meadows	Dec. 31, 2012 Joint venture 747 Madison Avenue	Dec. 31, 2011 Joint venture 747 Madison Avenue	Dec. 31, 2012 Joint venture West Coast office portfolio	Dec. 31, 2012 Joint venture 180/182 Broadway and 63 Nassu Street	Dec. 31, 2011 Joint venture 180/182 Broadway and 63 Nassu Street	Dec. 31, 2012 Joint venture 16 Court Street	Dec. 31, 2011 Joint venture 16 Court Street	Dec. 31, 2012 Joint venture 1552 Broadway	Dec. 31, 2011 Joint venture 1552 Broadway	Apr. 30, 2012 Joint venture 27-29 West 34th Street	Dec. 31, 2012 Joint venture 27-29 West 34th Street	Dec. 31, 2011 Joint venture 27-29 West 34th Street	Dec. 31, 2011 Joint venture 717 Fifth Avenue	Dec. 31, 2011 Joint venture 379 West Broadway	Dec. 31, 2012 Joint venture Other loan payable	Dec. 31, 2011 Joint venture Other loan payable
First mortgage notes and other loan payable
Interest rate, fixed rate debt (as a percent)									4.45%	9.00%				3.20%				3.04%	6.00%		5.65%		5.68%		5.76%		6.57%		4.82%			10.00%	6.64%		5.63%		5.66%
Interest rate, floating rate debt (as a percent)														1.36%																					2.88%	2.41%				2.32%		2.96%	2.71%	2.56%		2.45%			7.75%		3.00%		3.96%	2.96%		2.71%		3.21%			2.21%				1.11%
Total fixed rate debt	$ 3,457,769,000	$ 3,431,353,000							$ 300,000,000	$ 294,509,000	$ 3,219,885,000	$ 3,040,803,000		$ 996,082,000	$ 1,106,757,000			$ 25,000,000	$ 20,910,000	$ 20,910,000	$ 163,750,000	$ 163,750,000	$ 340,000,000	$ 340,000,000	$ 100,000,000	$ 100,000,000	$ 710,000,000	$ 710,000,000	$ 17,491,000	$ 17,761,000	$ 315,000,000	$ 856,000	$ 212,287,000	$ 214,625,000	$ 12,000,000		$ 27,000,000	$ 27,000,000	$ 25,000,000
Total floating rate debt	1,157,695,000	942,487,000		150,000,000		150,000,000					2,138,163,000	1,091,087,000		142,297,000	31,622,000	72,000,000	70,300,000																		2,033,000	170,000,000				124,384,000	125,000,000	18,362,000	125,000,000	120,000,000		247,253,000	254,896,000	60,000,000	57,000,000	84,698,000	33,125,000	33,125,000	745,025,000	71,524,000	30,722,000	84,916,000	85,728,000	113,869,000	95,405,000		53,375,000	53,900,000	245,000,000	20,991,000	30,000,000	30,000,000
Total mortgages and other loans payable	4,615,464,000	4,314,741,000									5,358,048,000	4,131,890,000				576,000,000	562,400,000
Committed amount																																										75,000,000												90,000,000				125,000,000
Ownership interest sold (as a percent)			49.50%																																	49.50%
Mortgage on properties assumed			170,000,000		140,000,000																															170,000,000
Maximum amount of loan recourse to entity											218,400,000						200,000,000
Refinanced mortgage loan							68,000,000																																						260,000,000
Portion of bridge loan provided to joint venture (as a percent)																																													50.00%
Term of refinanced mortgage			7 years	2 years				2 years																												7 years									5 years			3 years
Interest rate added to base rate (as a percent)			2.20%	2.00%																																										2.10%
Interest rate, description				30-Day LIBOR																																										30-day LIBOR
Possible increase in mortgage based on meeting certain performance hurdles																																														40,000,000
Management, leasing, construction supervision and asset management services revenue											7,900,000	8,600,000	5,800,000
Period for which the loan is extended																																																												1 year
Undrawn amount																																																	3,000,000
Gain (loss) on early extinguishment of debt																																																	$ 10,800,000