Investments in Unconsolidated Joint Ventures (Details 2) (USD $)								3 Months Ended		6 Months Ended																																					1 Months Ended			1 Months Ended	6 Months Ended		1 Months Ended			0 Months Ended	1 Months Ended	0 Months Ended	1 Months Ended															1 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 717 Fifth Avenue Mortgage loan	Dec. 31, 2012 717 Fifth Avenue Mortgage loan	Jun. 30, 2013 717 Fifth Avenue Mezzanine loans	Dec. 31, 2012 717 Fifth Avenue Mezzanine loans	Jun. 30, 2013 16 Court Street	Jun. 30, 2013 Joint venture	Jun. 30, 2012 Joint venture	Jun. 30, 2013 Joint venture	Jun. 30, 2012 Joint venture	Dec. 31, 2012 Joint venture	Jun. 30, 2013 Joint venture 100 Park Avenue	Dec. 31, 2012 Joint venture 100 Park Avenue	Jun. 30, 2013 Joint venture 7 Renaissance	Dec. 31, 2012 Joint venture 7 Renaissance	Jun. 30, 2013 Joint venture 11 West 34th Street	Dec. 31, 2012 Joint venture 11 West 34th Street	Jun. 30, 2013 Joint venture 280 Park Avenue	Dec. 31, 2012 Joint venture 280 Park Avenue	Jun. 30, 2013 Joint venture 21 West 34th Street	Dec. 31, 2012 Joint venture 21 West 34th Street	Jun. 30, 2013 Joint venture 1745 Broadway	Dec. 31, 2012 Joint venture 1745 Broadway	Jun. 30, 2013 Joint venture 1 and 2 Jericho Plaza	Dec. 31, 2012 Joint venture 1 and 2 Jericho Plaza	Jun. 30, 2013 Joint venture 800 Third Avenue	Dec. 31, 2012 Joint venture 800 Third Avenue	Jun. 30, 2013 Joint venture 388 and 390 Greenwich Street	Dec. 31, 2012 Joint venture 388 and 390 Greenwich Street	Jun. 30, 2013 Joint venture 388 and 390 Greenwich Street Mortgage loan	Mar. 31, 2013 Joint venture 388 and 390 Greenwich Street Mortgage loan	Jun. 30, 2013 Joint venture 388 and 390 Greenwich Street Mezzanine loans	Jun. 30, 2013 Joint venture 315 West 36th Street	Dec. 31, 2012 Joint venture 315 West 36th Street	Jun. 30, 2013 Joint venture 21 East 66th Street	Dec. 31, 2012 Joint venture 21 East 66th Street	Jun. 30, 2013 Joint venture 1604-1610 Broadway	Dec. 31, 2012 Joint venture 1604-1610 Broadway	Jun. 30, 2013 Joint venture 180/182 Broadway and 63 Nassu 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	Jun. 30, 2013 Joint venture West Coast office portfolio	Dec. 31, 2012 Joint venture West Coast office portfolio	Jun. 30, 2013 Joint venture 747 Madison Avenue	Dec. 31, 2012 Joint venture 747 Madison Avenue	Jun. 30, 2013 Joint venture The Meadows	Aug. 31, 2012 Joint venture The Meadows	Dec. 31, 2012 Joint venture The Meadows	Apr. 30, 2011 Joint venture 3 Columbus Circle	Jun. 30, 2013 Joint venture 3 Columbus Circle	Dec. 31, 2012 Joint venture 3 Columbus Circle	Apr. 30, 2013 Joint venture 1552 Broadway	Jun. 30, 2013 Joint venture 1552 Broadway	Dec. 31, 2012 Joint venture 1552 Broadway	Jun. 30, 2013 Joint venture 1552 Broadway Mortgage loan	Apr. 30, 2013 Joint venture 1552 Broadway Mortgage loan	Jun. 30, 2013 Joint venture 1552 Broadway Mezzanine loans	Apr. 30, 2013 Joint venture 1552 Broadway Mezzanine loans	Apr. 30, 2013 Joint venture 1552 Broadway Construction financing facility item	Jun. 30, 2013 Joint venture Other loan payable	Dec. 31, 2012 Joint venture Other loan payable	Jun. 30, 2013 Joint venture 724 Fifth Avenue	Dec. 31, 2012 Joint venture 724 Fifth Avenue	Jun. 30, 2013 Joint venture 10 East 53rd Street	Dec. 31, 2012 Joint venture 10 East 53rd Street	Jun. 30, 2013 Joint venture 33 Beekman	Mar. 31, 2013 Joint venture 33 Beekman	Dec. 31, 2012 Joint venture 33 Beekman	Jun. 30, 2013 Joint venture 600 Lexington Avenue	Dec. 31, 2012 Joint venture 600 Lexington Avenue	Jun. 30, 2013 Joint venture 27-29 West 34th Street	Dec. 31, 2012 Joint venture 27-29 West 34th Street	Nov. 30, 2012 Joint venture 521 Fifth Avenue	Jun. 30, 2013 Joint venture 521 Fifth Avenue	Dec. 31, 2012 Joint venture 521 Fifth Avenue	Jun. 30, 2013 Joint venture 16 Court Street	Dec. 31, 2012 Joint venture 16 Court Street
First mortgage notes and other loan payable
Interest rate, fixed rate debt (as a percent)			4.45%		9.00%								6.64%		10.00%		4.82%		6.57%		5.76%		5.68%		5.65%		6.00%		3.20%					3.04%		3.60%		5.66%
Interest rate, floating rate debt (as a percent)							3.44%																						1.19%							1.90%				2.95%			3.93%		3.00%		7.75%				2.38%			3.23%							1.10%		2.55%		2.70%		2.95%			2.28%		2.10%			2.40%		3.44%
Total fixed rate debt	$ 4,340,978,000	$ 3,457,769,000	$ 300,000,000	$ 300,000,000	$ 299,122,000	$ 294,509,000		$ 3,223,219,000		$ 3,223,219,000		$ 3,219,885,000	$ 211,056,000	$ 212,287,000	$ 950,000	$ 856,000	$ 17,349,000	$ 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	$ 12,000,000	$ 12,000,000	$ 27,000,000	$ 27,000,000
Total floating rate debt	505,753,000	1,157,695,000					84,580,000	1,971,990,000		1,971,990,000		2,138,163,000																	142,297,000	142,297,000	72,000,000		70,300,000			1,996,000	2,033,000			88,321,000		71,524,000	635,012,000	745,025,000	33,125,000	33,125,000	57,000,000		57,000,000		243,277,000	247,253,000		131,848,000	113,869,000						30,000,000	30,000,000	120,000,000	120,000,000	125,000,000	125,000,000	18,362,000		18,362,000	122,514,000	124,384,000	53,238,000	53,375,000		170,000,000	170,000,000		84,916,000
Amount of original debt being refinanced into another loan																																																83,000,000					119,600,000
Total joint venture mortgages and other loans payable	4,835,231,000	4,615,464,000						5,195,209,000		5,195,209,000		5,358,048,000																			576,000,000	576,000,000	562,400,000
Committed amount																																									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																																																60,000,000		260,000,000							170,000,000		30,000,000	200,000,000
Unfunded amount																																															3,000,000									47,300,000		20,900,000
Term of refinanced mortgage																																																3 years		5 years										3 years														7 years
Interest rate added to base rate (as a percent)																																																		2.10%							2.70%		9.35%
Interest rate, description																																																		30-day LIBOR							LIBOR		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								1,400,000	1,800,000	4,000,000	3,700,000
Gain (loss) on early extinguishment of debt																																																$ 10,800,000