Investments in Unconsolidated Joint Ventures (Details 2) (USD $)							3 Months Ended																																	1 Months Ended											1 Months Ended			1 Months Ended	3 Months Ended												1 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 717 Fifth Avenue Mortgage loan	Dec. 31, 2012 717 Fifth Avenue Mortgage loan	Mar. 31, 2013 717 Fifth Avenue Mezzanine loans	Dec. 31, 2012 717 Fifth Avenue Mezzanine loans	Mar. 31, 2013 Joint venture	Mar. 31, 2012 Joint venture	Dec. 31, 2012 Joint venture	Mar. 31, 2013 Joint venture 21 East 66th Street	Dec. 31, 2012 Joint venture 21 East 66th Street	Mar. 31, 2013 Joint venture 100 Park Avenue	Dec. 31, 2012 Joint venture 100 Park Avenue	Mar. 31, 2013 Joint venture 7 Renaissance	Dec. 31, 2012 Joint venture 7 Renaissance	Mar. 31, 2013 Joint venture 11 West 34th Street	Dec. 31, 2012 Joint venture 11 West 34th Street	Mar. 31, 2013 Joint venture 280 Park Avenue	Dec. 31, 2012 Joint venture 280 Park Avenue	Mar. 31, 2013 Joint venture 21 West 34th Street	Dec. 31, 2012 Joint venture 21 West 34th Street	Mar. 31, 2013 Joint venture 1745 Broadway	Dec. 31, 2012 Joint venture 1745 Broadway	Mar. 31, 2013 Joint venture 1 and 2 Jericho Plaza	Dec. 31, 2012 Joint venture 1 and 2 Jericho Plaza	Mar. 31, 2013 Joint venture 800 Third Avenue	Dec. 31, 2012 Joint venture 800 Third Avenue	Mar. 31, 2013 Joint venture 388 and 390 Greenwich Street	Dec. 31, 2012 Joint venture 388 and 390 Greenwich Street	Mar. 31, 2013 Joint venture 388 and 390 Greenwich Street Mortgage loan	Mar. 31, 2013 Joint venture 388 and 390 Greenwich Street Mezzanine loans	Mar. 31, 2013 Joint venture 315 West 36th Street	Dec. 31, 2012 Joint venture 315 West 36th Street	Mar. 31, 2013 Joint venture 1604-1610 Broadway	Dec. 31, 2012 Joint venture 1604-1610 Broadway	Mar. 31, 2013 Joint venture 27-29 West 34th Street	Dec. 31, 2012 Joint venture 27-29 West 34th Street	Mar. 31, 2013 Joint venture 1552 Broadway	Dec. 31, 2012 Joint venture 1552 Broadway	Apr. 30, 2013 Joint venture 1552 Broadway Mortgage loan	Apr. 30, 2013 Joint venture 1552 Broadway Mezzanine loans	Apr. 30, 2013 Joint venture 1552 Broadway Mortgage and Mezzanine loans item	Mar. 31, 2013 Joint venture 16 Court Street	Dec. 31, 2012 Joint venture 16 Court Street	Mar. 31, 2013 Joint venture 180/182 Broadway and 63 Nassu Street	Dec. 31, 2012 Joint venture 180/182 Broadway and 63 Nassu Street	Mar. 31, 2013 Joint venture West Coast office portfolio	Dec. 31, 2012 Joint venture West Coast office portfolio	Mar. 31, 2013 Joint venture 747 Madison Avenue	Dec. 31, 2012 Joint venture 747 Madison Avenue	Aug. 31, 2012 Joint venture The Meadows	Mar. 31, 2013 Joint venture The Meadows	Dec. 31, 2012 Joint venture The Meadows	Apr. 30, 2011 Joint venture 3 Columbus Circle	Mar. 31, 2013 Joint venture 3 Columbus Circle	Dec. 31, 2012 Joint venture 3 Columbus Circle	Mar. 31, 2013 Joint venture Other loan payable	Dec. 31, 2012 Joint venture Other loan payable	Mar. 31, 2013 Joint venture 724 Fifth Avenue	Dec. 31, 2012 Joint venture 724 Fifth Avenue	Mar. 31, 2013 Joint venture 10 East 53rd Street	Dec. 31, 2012 Joint venture 10 East 53rd Street	Mar. 31, 2013 Joint venture 33 Beekman	Dec. 31, 2012 Joint venture 33 Beekman	Mar. 31, 2013 Joint venture 600 Lexington Avenue	Dec. 31, 2012 Joint venture 600 Lexington Avenue	Nov. 30, 2012 Joint venture 521 Fifth Avenue	Mar. 31, 2013 Joint venture 521 Fifth Avenue
First mortgage notes and other loan payable
Interest rate, fixed rate debt (as a percent)			4.45%		9.00%					5.63%		6.64%		10.00%		4.82%		6.57%		5.76%		5.68%		5.65%		6.00%		3.20%				3.04%		5.66%
Interest rate, floating rate debt (as a percent)										2.88%																		1.36%								2.20%		3.18%					2.70%		2.96%		3.94%		3.00%			7.75%			2.41%		1.10%		2.56%		2.71%		2.96%		2.31%			2.41%
Total fixed rate debt	$ 4,349,406,000	$ 3,457,769,000	$ 300,000,000	$ 300,000,000	$ 296,803,000	$ 294,509,000	$ 3,221,492,000		$ 3,219,885,000	$ 12,000,000	$ 12,000,000	$ 211,673,000	$ 212,287,000	$ 856,000	$ 856,000	$ 17,418,000	$ 17,491,000	$ 710,000,000	$ 710,000,000	$ 100,000,000	$ 100,000,000	$ 340,000,000	$ 340,000,000	$ 163,750,000	$ 163,750,000	$ 20,910,000	$ 20,910,000	$ 996,082,000	$ 996,082,000			$ 25,000,000	$ 25,000,000	$ 27,000,000	$ 27,000,000
Total floating rate debt	466,079,000	1,157,695,000					2,142,811,000		2,138,163,000	2,033,000	2,033,000																	142,297,000	142,297,000	72,000,000	70,300,000					53,238,000	53,375,000	119,322,000	113,869,000				84,731,000	84,916,000	76,862,000	71,524,000	742,112,000	745,025,000	33,125,000	33,125,000		57,000,000	57,000,000		245,275,000	247,253,000	30,000,000	30,000,000	120,000,000	120,000,000	125,000,000	125,000,000	18,362,000	18,362,000	123,454,000	124,384,000		170,000,000
Total mortgages and other loans payable	4,815,485,000	4,615,464,000					5,364,303,000		5,358,048,000																					576,000,000	562,400,000
Committed amount																																						125,000,000							90,000,000																		75,000,000
Ownership interest sold (as a percent)																																																																			49.50%
Mortgage on properties assumed																																																																			170,000,000
Maximum amount of loan recourse to entity																															200,000,000
Refinanced mortgage loan																																								170,000,000	30,000,000	200,000,000									60,000,000			260,000,000
Term of refinanced mortgage																																										3 years									3 years			5 years													7 years
Interest rate added to base rate (as a percent)																																								2.70%	9.35%													2.10%													2.20%
Interest rate, description																																								LIBOR	LIBOR													30-day LIBOR
Number of extension options																																										2
Extension option term																																										1 year
Possible increase in mortgage based on meeting certain performance hurdles																																																							40,000,000
Management, leasing, construction supervision and asset management services revenue							2,600,000	1,900,000
Gain (loss) on early extinguishment of debt																																																			10,800,000
Undrawn amount																																																				$ 3,000,000