Mortgages and Other Loans Payable (Details) (USD $)	3 Months Ended			12 Months Ended				1 Months Ended				12 Months Ended				1 Months Ended																			1 Months Ended													12 Months Ended			1 Months Ended		1 Months Ended		1 Months Ended			1 Months Ended					1 Months Ended
	Dec. 31, 2012	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Series E preferred units	Sep. 30, 2012 Master Repurchase Agreement	Dec. 31, 2012 Master Repurchase Agreement	Dec. 31, 2012 Other loan payable due in September, 2019	Dec. 31, 2011 Other loan payable due in September, 2019	Dec. 31, 2012 Other loan payable due in June 2013	Dec. 31, 2011 Other loan payable due in June 2013	Dec. 31, 2012 400 East 57th Street	Dec. 31, 2012 400 East 58th Street	Jun. 30, 2011 919 Third Avenue	Dec. 31, 2012 919 Third Avenue	Dec. 31, 2011 919 Third Avenue	Dec. 31, 2012 100 Church	Dec. 31, 2012 One Madison Avenue	Dec. 31, 2011 One Madison Avenue	Dec. 31, 2012 885 Third Avenue	Dec. 31, 2011 885 Third Avenue	Dec. 31, 2012 2 Herald Square	Dec. 31, 2011 2 Herald Square	Dec. 31, 2012 485 Lexington Avenue	Dec. 31, 2011 485 Lexington Avenue	Dec. 31, 2012 120 West 45th Street	Dec. 31, 2011 120 West 45th Street	Dec. 31, 2012 300 Main Street	Dec. 31, 2011 300 Main Street	Dec. 31, 2012 762 Madison Avenue	Dec. 31, 2012 Landmark Square	Dec. 31, 2011 Landmark Square	Mar. 31, 2011 420 Lexington Avenue	Dec. 31, 2012 420 Lexington Avenue	Dec. 31, 2011 420 Lexington Avenue	Dec. 31, 2012 500 West Putnam	Dec. 31, 2011 500 West Putnam	Dec. 31, 2012 625 Madison Avenue	Dec. 31, 2011 625 Madison Avenue	Dec. 31, 2012 711 Third Avenue	Dec. 31, 2011 711 Third Avenue	Dec. 31, 2012 125 Park Avenue	Dec. 31, 2011 125 Park Avenue	Dec. 31, 2012 609 Partners, LLC	Dec. 31, 2011 609 Partners, LLC	Dec. 31, 2012 609 Partners, LLC Series E preferred units	Dec. 31, 2012 220 East 42nd Street	Dec. 31, 2011 220 East 42nd Street	Dec. 31, 2012 609 Fifth Avenue	Dec. 31, 2011 609 Fifth Avenue	Nov. 30, 2012 673 First Avenue	Dec. 31, 2011 673 First Avenue	Dec. 31, 2012 110 East 42nd Street	Dec. 31, 2011 110 East 42nd Street	Dec. 31, 2011 292 Madison Avenue	Apr. 30, 2012 1515 Broadway	Dec. 31, 2012 1515 Broadway	Dec. 31, 2011 1515 Broadway	Dec. 31, 2012 180 Maiden Lane	Dec. 31, 2011 180 Maiden Lane	Nov. 30, 2012 521 Fifth Avenue	Apr. 30, 2011 521 Fifth Avenue	Dec. 31, 2011 521 Fifth Avenue	Jan. 31, 2011 521 Fifth Avenue
Mortgages and Other Loans Payable
Interest rate, fixed rate debt (as a percent)										8.00%				4.13%	4.13%	6.87%	5.12%		4.68%	5.91%		6.26%		5.36%		5.61%		6.12%		5.75%		3.75%	4.00%			7.50%		5.52%		7.22%		4.99%		5.75%		5.00%			5.25%		5.85%		5.67%		5.81%
Interest rate, floating rate debt (as a percent)									3.21%																																																		3.60%		2.42%
Total fixed rate debt	$ 3,457,769,000			$ 3,457,769,000	$ 3,431,353,000					$ 50,000,000	$ 50,000,000			$ 70,000,000	$ 30,000,000		$ 500,000,000	$ 500,000,000	$ 230,000,000	$ 607,678,000	$ 626,740,000	$ 267,650,000	$ 267,650,000	$ 191,250,000	$ 191,250,000	$ 450,000,000	$ 450,000,000	$ 170,000,000	$ 170,000,000	$ 11,500,000	$ 11,500,000	$ 8,371,000	$ 84,486,000	$ 86,000,000		$ 184,992,000	$ 187,182,000	$ 24,060,000	$ 24,563,000	$ 125,603,000	$ 129,098,000	$ 120,000,000	$ 120,000,000	$ 146,250,000	$ 146,250,000	$ 23,000	$ 31,721,000		$ 185,906,000	$ 190,431,000		$ 94,963,000		$ 29,906,000		$ 65,000,000	$ 59,099,000
Total floating rate debt	1,157,695,000			1,157,695,000	942,487,000				116,667,000				62,792,000																																														769,813,000	450,363,000	271,215,000	279,332,000			150,000,000
Total mortgages and other loans payable	4,615,464,000			4,615,464,000	4,373,840,000
Mortgage on property assumed at acquisition																																																																		140,000,000
Term																																																										7 years					7 years	2 years
Interest rate added to base rate (as a percent)								3.00%				2.50%																																														2.85%					2.20%	2.00%
Interest rate, description								1-month LIBOR				three month GBP LIBOR																																														90-day LIBOR					LIBOR	30-day LIBOR
Controlling interest in the joint venture (as a percent)																	51.00%																																												49.90%
Repaid loan																219,900,000																																			93,300,000		29,100,000		65,000,000			447,200,000
Refinanced mortgage																500,000,000																																										775,000,000						150,000,000
Loan B-tranche amount																																																								18,100,000
Increase in loan amount																																			40,000,000																												170,000,000
Preferred units, shares issued																																																63,900,000
Preferred units (as a percent)							5.00%																																									5.00%
Preferred units, liquidation preference (in dollars per share)																																																$ 1.00
Preferred units, shares redeemed																																																63,800,000
Variable interest rate floor (as a percent)																																																										3.60%
Maximum facility capacity								175,000,000
Extension option available for debt instrument								1 year
Partnership interest sold (as a percent)																																																															49.50%
Loss on early extinguishment of debt	6,978,000	67,000	(971,000)	6,978,000	(904,000)	1,900,000																																													3,100,000
Gross book value of the properties collateralizing the mortgages and other loans payable	$ 7,600,000,000			$ 7,600,000,000	$ 7,400,000,000