Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended							12 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			12 Months Ended			12 Months Ended		1 Months Ended
	Dec. 31, 2012 item sqft	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2012 item sqft	Dec. 31, 2011	Dec. 31, 2010	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	Dec. 31, 2012 717 Fifth Avenue sqft	Dec. 31, 2012 800 Third Avenue sqft	Jul. 31, 2012 One Court Square	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, 2012 The Meadows sqft	Dec. 31, 2010 The Meadows	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 item	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	Sep. 30, 2012 3 Columbus Circle	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 sqft item	Apr. 30, 2012 379 West Broadway	Mar. 31, 2012 141 Fifth Avenue	Nov. 30, 2011 450 West 33rd Street	Aug. 31, 2011 1551-1555 Broadway	Dec. 31, 2012 1552 Broadway sqft	Sep. 30, 2012 West Coast office portfolio item	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 and 2 residential units	Dec. 31, 2012 21 East 66th Street Three retail units item	Dec. 31, 2012 21 East 66th Street Two residential units item	Dec. 31, 2012 21 East 66th Street Four residential units item	Dec. 31, 2012 315 West 36th Street sqft	May 31, 2010 Green Hill Acquisition LLC 1221 Avenue of the Americas
Investment in Unconsolidated Joint Ventures
Number of VIEs in which the entity is not the primary beneficiary								3
Net equity investment in VIEs in which the entity is not the primary beneficiary	$ 117,700,000				$ 161,900,000			$ 117,700,000	$ 161,900,000
General information on each joint venture
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%	30.00%	50.00%			48.90%		50.00%	50.00%	50.00%		33.33%	50.00%	55.00%	45.90%			100.00%				27.63%		50.50%	32.28%					35.50%
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			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					13,045		4,474,000		460,000	17,000					148,000
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
Remaining percentage of interest acquired in joint venture																					50.00%																									50.00%
Depreciable real estate reserve																							2,800,000
Triple-net lease arrangement																								13 years
Number of floors of student housing																										20																	30
Number of properties	66							66																																									31						3	2	4
Prior to the recapitalization positions held in mezzanine and preferred equity																																																	26,700,000
Debt issued																																																	678,800,000		170,000,000
Term																																																	2 years		7 years
Extension option																																																	1 year
Contribution made by Harel																										28,100,000
Ownership sold																										49.00%																									49.50%
Sales price															617,600,000		481,100,000								2,800,000							143,600,000												48,500,000	46,000,000						315,000,000
Repayment of debt								688,065,000	765,378,000	149,832,000																			12,000,000
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
Committed 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																																				111,300,000
Floating rate financing assumed by joint venture									30,000,000								315,000,000																			30,000,000
Valuation of investment under the recapitalization transaction	10,269,630,000				10,010,548,000			10,269,630,000	10,010,548,000
Valuation of investment under the recapitalization transaction																																			1,100,000,000
Effective ownership interest in underlying investment (as a percent)																																			49.50%																			32.28%			16.14%
Fees position acquired																																												13,500,000
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		1,000,000																											6,500,000	7,300,000		4,000,000				19,400,000								126,800,000
Fixed rate debt	3,457,769,000				3,431,353,000			3,457,769,000	3,431,353,000
Beneficial interest sold (as a percent)															50.00%																																10.00%												45.00%
Consideration received for sale of beneficial interest																																															9,700,000												577,400,000
Additional cash income recognized due to the distribution of refinancing proceeds															67,900,000
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																														1,500,000						1,000,000								4,500,000
Existing loan repaid																																																			$ 150,000,000
Interest rate, basis points																																																			2.20%
Interest rate, description																																																			LIBOR