Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended							12 Months Ended								1 Months Ended						1 Months Ended	12 Months Ended			1 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 sqft property	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2012 entity sqft property	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Residential properties sqft property	Dec. 31, 2012 100 Park Avenue sqft	Dec. 31, 2012 21 West 34th Street sqft	Dec. 31, 2012 1604-1610 Broadway sqft	Dec. 31, 2012 27-29 West 34th Street sqft	Jun. 30, 2012 717 Fifth Avenue	Dec. 31, 2012 717 Fifth Avenue sqft	Dec. 31, 2012 800 Third Avenue sqft	Dec. 31, 2012 1745 Broadway sqft	Dec. 31, 2012 1 and 2 Jericho Plaza sqft	Dec. 31, 2012 16 Court Street sqft	Sep. 30, 2009 The Meadows	Dec. 31, 2010 The Meadows	Dec. 31, 2012 The Meadows sqft	Dec. 31, 2012 388 and 390 Greenwich Street sqft	Aug. 31, 2011 180/182 Broadway and 63 Nassu Street	Dec. 31, 2010 180/182 Broadway and 63 Nassu Street floor	Dec. 31, 2012 180/182 Broadway and 63 Nassu Street sqft	Dec. 31, 2012 600 Lexington Avenue sqft	Dec. 31, 2010 11 West 34th Street	Dec. 31, 2012 11 West 34th Street sqft	Dec. 31, 2012 7 Renaissance sqft	Apr. 30, 2011 3 Columbus Circle	Dec. 31, 2012 3 Columbus Circle sqft	May 31, 2011 280 Park Avenue	Mar. 31, 2011 280 Park Avenue	Dec. 31, 2012 280 Park Avenue sqft	Dec. 31, 2012 1552-1560 Broadway sqft	May 31, 2012 155 West 46th Street	Dec. 31, 2012 747 Madison Avenue sqft	Dec. 31, 2012 724 Fifth Avenue sqft	Dec. 31, 2012 10 East 53rd Street sqft	Dec. 31, 2012 33 Beekman floor sqft	Sep. 30, 2012 West Coast office portfolio property	Dec. 31, 2012 West Coast office portfolio sqft	Nov. 30, 2012 521 Fifth Avenue	Dec. 31, 2012 521 Fifth Avenue sqft	Dec. 31, 2012 21 East 66th Street sqft	Dec. 31, 2012 21 East 66th Street Three retail units unit	Dec. 31, 2012 21 East 66th Street 2 residential units unit	Dec. 31, 2012 21 East 66th Street Four residential units unit	Dec. 31, 2012 315 West 36th Street sqft	Jul. 31, 2012 One Court Square	Mar. 31, 2012 141 Fifth Avenue	Apr. 30, 2012 379 West Broadway	Aug. 31, 2011 1551-1555 Broadway	Dec. 31, 2012 1552 Broadway sqft	Nov. 30, 2011 450 West 33rd Street	May 31, 2010 Green Hill Acquisition LLC 1221 Avenue of the Americas
General information on each joint venture
Number of VIEs in which the entity is not primary beneficiary								3
Net equity investment in VIEs in which the entity is not primary beneficiary	$ 117,700,000				$ 161,900,000			$ 117,700,000	$ 161,900,000
Beneficial interest sold (as a percent)																50.00%																																								10.00%			45.00%
Consideration received for sale of beneficial interest																																		143,600,000																						9,700,000			577,400,000
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
Employee compensation award																1,000,000																														1,000,000								1,500,000					4,500,000
Ownership Interest (as a percent)												49.90%	50.00%	45.00%	50.00%		10.92%	42.95%	32.26%	20.26%	35.00%			50.00%	50.60%			25.50%	55.00%		30.00%	50.00%		48.90%			50.00%	50.00%		33.33%	50.00%	55.00%	45.90%		27.63%		50.50%	32.28%	32.28%		16.14%	35.50%						100.00%
Economic Interest (as a percent)												49.90%	50.00%	63.00%	50.00%		10.92%	42.95%	32.26%	20.26%	35.00%			50.00%	50.60%			25.50%	55.00%		30.00%	50.00%		48.90%			50.00%	50.00%		33.33%	50.00%	55.00%	45.90%		27.63%		50.50%	32.28%				35.50%
Square Feet	29,685,079							29,685,079			430,482	834,000	30,000	30,000	41,000		120,000	526,000	674,000	640,000	318,000			582,000	2,600,000			71,000	304,000		17,000	37,000		769,000			1,237,000	49,000		10,000	65,000	390,000	145,000		4,474,000		460,000	17,000				148,000					13,045
Acquisition Price												95,800,000	22,400,000	4,400,000	30,000,000		251,900,000	285,000,000	520,000,000	210,000,000	107,500,000			111,500,000	1,575,000,000			43,600,000	193,000,000		10,800,000	4,000,000		500,000,000			400,000,000	136,550,000	8,400,000	66,250,000	223,000,000	252,500,000	31,000,000		880,103,000		315,000,000	75,000,000				45,000,000
Depreciable real estate reserves					(5,789,000)				(5,789,000)	(2,750,000)													2,800,000
Fees position acquired																																																							13,500,000
Remaining percentage of interest acquired in joint venture																						50.00%																																				50.00%
Triple-net lease arrangement																									13 years
Number of floors of student housing																											20																30
Contribution made by Harel																											28,100,000
Partnership interest sold to Harel by joint venture (as a percent)																											49.00%
Sales price																617,600,000										2,800,000																				315,000,000							481,100,000	46,000,000	48,500,000
Repayment of debt								688,065,000	765,378,000	149,832,000																				12,000,000
Additional cash income recognized due to distribution of refinancing proceeds																67,900,000
Additional income recognized on repayment of debt	(6,978,000)					(67,000)	971,000	(6,978,000)	904,000	(1,900,000)																				1,100,000
Operating partnership units issued (in shares)																																		306,296
Obligation to fund additional contribution																																		47,500,000
Committed additional capital contribution funded																																		46,800,000
Bridge loan made																																	125,000,000
Interest rate on bridge loan made (as a percent)																																	7.50%
Debt investments contributed to joint venture								25,362,000	286,571,000																											286,600,000
Additional income upon the contribution of debt investment to joint venture								119,155,000	120,418,000	147,926,000																										38,700,000
Amount received from joint venture								178,485,000	160,548,000	623,121,000																										111,300,000
Prior to the recapitalization positions held in mezzanine and preferred equity																																												26,700,000
Increase in loan amount																																												678,800,000
Term																																												2 years		7 years
Ownership interest sold (as a percent)																											49.00%
Refinanced mortgage loan																																														150,000,000
Mortgage on properties assumed																																														170,000,000
Interest rate, description																																														LIBOR
Number of units in property																																																	3	2	4
Period of extension option for mortgage secured by the portfolio																																												1 year
Floating rate financing assumed by joint venture									30,000,000																											30,000,000																	315,000,000
Valuation of investment under the recapitalization transaction																																			1,100,000,000
Effective ownership interest in underlying investment (as a percent)																																			49.50%
Recognized gain (loss) on sale of interest in property	$ 19,277,000	$ (4,807,000)	$ 15,323,000	$ 7,260,000	$ (114,000)	$ 3,032,000		$ 37,053,000	$ 2,918,000	$ 128,921,000						$ 3,000,000																														$ 19,400,000							$ 1,000,000	$ 7,300,000	$ 6,500,000	$ 4,000,000			$ 126,800,000
Number of Properties	66							66			2																																	31