Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended							12 Months Ended												1 Months Ended	12 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended		12 Months Ended		12 Months Ended	1 Months Ended				1 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011 entity sqft	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2011 100 Park Avenue sqft	Dec. 31, 2011 379 West Broadway sqft	Dec. 31, 2011 21 West 34th Street sqft	Dec. 31, 2011 800 Third Avenue sqft	Dec. 31, 2011 One Court Square sqft	Dec. 31, 2011 1604-1610 Broadway sqft	Dec. 31, 2011 1745 Broadway sqft	Dec. 31, 2011 1 and 2 Jericho Plaza sqft	Dec. 31, 2011 16 Court Street sqft	Sep. 30, 2009 The Meadows	Dec. 31, 2010 The Meadows sqft	Dec. 31, 2011 The Meadows	Dec. 31, 2011 388 and 390 Greenwich Street sqft y	Dec. 31, 2011 27-29 West 34th Street sqft	Aug. 31, 2011 1551-1555 Broadway	Dec. 31, 2011 717 Fifth Avenue sqft	Dec. 31, 2011 141 Fifth Avenue sqft	Aug. 31, 2011 180/182 Broadway and 63 Nassu Street	Dec. 31, 2010 180/182 Broadway and 63 Nassu Street floor	Dec. 31, 2011 180/182 Broadway and 63 Nassu Street sqft floor	Dec. 31, 2011 600 Lexington Avenue sqft	Dec. 31, 2010 11 West 34th Street	Dec. 31, 2011 11 West 34th Street sqft	Dec. 31, 2011 7 Renaissance sqft	Dec. 31, 2011 3 Columbus Circle sqft	Mar. 31, 2011 280 Park Avenue	Dec. 31, 2011 280 Park Avenue sqft	May 31, 2011 280 Park Avenue	Dec. 31, 2011 1552-1560 Broadway sqft	Dec. 31, 2011 747 Madison Avenue	Dec. 31, 2011 1552 Broadway sqft	Apr. 30, 2009 55 Corporate Drive, NJ (pad IV)	Jun. 30, 2009 1166 Avenue of the Americas	Nov. 30, 2011 450 West 33rd Street	Dec. 31, 2011 450 West 33rd Street	May 31, 2010 Green Hill Acquisition LLC 1221 Avenue of the Americas	Apr. 30, 2009 Mack-Green joint venture
General information on each joint venture
Number of VIEs in which the entity is not primary beneficiary								2
Net equity investment in VIEs in which the entity is not primary beneficiary																													$ 12,000,000						$ 161,900,000
Beneficial interest sold (as a percent)																									10.00%																	50.00%				45.00%
Consideration received for sale of beneficial interest																									9,700,000																		5,000,000			577,400,000
Proceeds from sale of interest in joint venture																																															500,000
Consideration towards existing reserve payment and assumption of pro-rata share of in-place financing																																														95,900,000
Proceeds from sale of beneficial interest in property																																														500,900,000
Recognized gain (loss) on sale of interest in property	(114,000)	3,032,000		1,633,000	520,000	126,769,000		2,918,000	128,921,000	6,691,000															4,000,000																	4,000,000	(5,200,000)			126,800,000
Employee compensation award																									2,500,000																					4,500,000
Ownership Interest (as a percent)											49.90%	45.00%	50.00%	42.95%	30.00%	45.00%	32.26%	20.26%	35.00%			50.00%	50.60%	50.00%		32.75%	50.00%			25.50%	55.00%		30.00%	50.00%	48.90%		50.00%		50.00%	33.33%					100.00%
Economic Interest (as a percent)											49.90%	45.00%	50.00%	42.95%	30.00%	63.00%	32.26%	20.26%	35.00%			50.00%	50.60%	50.00%		32.75%	50.00%			25.50%	55.00%		30.00%	50.00%	48.90%		50.00%		50.00%	33.33%
Square Feet								31,426,318			834,000	62,000	30,000	526,000	1,402,000	30,000	674,000	640,000	318,000		582,000		2,600,000	41,000		120,000	22,000			71,000	304,000		17,000	37,000	769,000		1,237,000		49,000		13,045
Acquisition Price											95,800,000	19,750,000	22,400,000	285,000,000	533,500,000	4,400,000	520,000,000	210,000,000	107,500,000			111,500,000	1,575,000,000	30,000,000		251,900,000	13,250,000			43,600,000	193,000,000		10,800,000	4,000,000	500,000,000		400,000,000		136,550,000	66,250,000
Remaining percentage of interest acquired in joint venture																				50.00%																								50.00%
Triple-net lease arrangement (in years)																							13
Adjustment to retained earnings due to deconsolidation of joint ventures																														3,000,000
Adjustment to noncontrolling interests in other partnerships due to deconsolidation of joint ventures																														9,500,000
Number of floors of student housing																													20
Contribution made by Harel																													28,100,000
Partnership interest sold to Harel by joint venture (as a percent)																													49.00%
Sales price															475,000,000													2,800,000
Repayment of debt								765,378,000	149,832,000	169,688,000																						12,000,000
Impairment charge in connection with expected sale															5,800,000						2,800,000
Additional income recognized on repayment of debt		(67,000)	971,000		(511,000)	(1,276,000)	(113,000)	904,000	(1,900,000)	86,006,000																						1,100,000
Operating partnership units issued (in shares)																																			306,296
Committed additional contribution																																			47,500,000
Committed additional capital contribution funded																																			34,500,000
Bridge loan made																																			125,000,000
Interest rate on bridge loan made (as a percent)																																			7.50%
Debt investments contributed to joint venture								286,571,000																												286,600,000
Additional income upon the contribution of debt investment to joint venture								120,418,000	147,926,000	65,608,000																										38,700,000
Amount received from joint venture								160,548,000	623,121,000	27,946,000																										111,300,000						4,500,000
Floating rate financing assumed by joint venture								30,000,000																												30,000,000
Valuation of investment under the recapitalization transaction																																						$ 1,100,000,000
Effective ownership interest in underlying investment (as a percent)																																				49.50%