Investment in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended				1 Months Ended	6 Months Ended		1 Months Ended		6 Months Ended				1 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	1 Months Ended		6 Months Ended		1 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended	1 Months Ended		6 Months Ended			6 Months Ended			1 Months Ended			6 Months Ended	1 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012 entity sqft	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 100 Park Avenue sqft	Jun. 30, 2012 21 West 34th Street sqft	Jun. 30, 2012 1604-1610 Broadway sqft	Jun. 30, 2012 27-29 West 34th Street sqft	Jun. 30, 2012 717 Fifth Avenue	Jun. 30, 2012 717 Fifth Avenue sqft	Jun. 30, 2012 800 Third Avenue sqft	Jul. 31, 2012 One Court Square	Nov. 30, 2011 One Court Square	Jun. 30, 2012 One Court Square sqft	Jun. 30, 2012 1745 Broadway sqft	Jun. 30, 2012 1 and 2 Jericho Plaza sqft	Jun. 30, 2012 16 Court Street sqft	Sep. 30, 2009 The Meadows	Jun. 30, 2012 The Meadows sqft	Dec. 31, 2010 The Meadows	Jun. 30, 2012 388 and 390 Greenwich Street sqft y	Aug. 31, 2011 180/182 Broadway and 63 Nassu Street	Dec. 31, 2010 180/182 Broadway and 63 Nassu Street floor	Jun. 30, 2012 180/182 Broadway and 63 Nassu Street sqft	Jun. 30, 2012 600 Lexington Avenue sqft	Dec. 31, 2010 11 West 34th Street	Jun. 30, 2012 11 West 34th Street sqft	Jun. 30, 2012 7 Renaissance sqft	Apr. 30, 2011 3 Columbus Circle	Jun. 30, 2012 3 Columbus Circle sqft	May 31, 2011 280 Park Avenue	Mar. 31, 2011 280 Park Avenue	Jun. 30, 2012 280 Park Avenue sqft	Jun. 30, 2012 1552-1560 Broadway sqft	May 31, 2012 155 West 46th Street	Jun. 30, 2012 747 Madison Avenue sqft	Jun. 30, 2012 724 Fifth Avenue sqft	Jun. 30, 2012 10 East 53rd Street sqft	Mar. 31, 2012 141 Fifth Avenue	Apr. 30, 2012 379 West Broadway	Aug. 31, 2011 1551-1555 Broadway	Jun. 30, 2012 1552 Broadway sqft	Nov. 30, 2011 450 West 33rd Street
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	$ 167,200,000		$ 167,200,000		$ 161,900,000
Beneficial interest sold (as a percent)										50.00%																																10.00%
Consideration received for sale of beneficial interest																																										9,700,000
Ownership Interest (as a percent)						49.90%	50.00%	45.00%	50.00%	10.92%	10.92%	42.95%			30.00%	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%					100.00%
Economic Interest (as a percent)						49.90%	50.00%	63.00%	50.00%	10.92%	10.92%	42.95%			30.00%	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%
Square Feet			30,776,460			834,000	30,000	30,000	41,000		120,000	526,000			1,402,000	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				13,045
Acquisition Price						95,800,000	22,400,000	4,400,000	30,000,000	251,900,000	251,900,000	285,000,000			533,500,000	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
Depreciable real estate reserves, net of recoveries	5,789,000		5,789,000																		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 (in years)																						13
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										617,600,000			481,100,000	475,600,000									2,800,000																	46,000,000	48,500,000
Repayment of debt			472,288,000	744,873,000																							12,000,000
Impairment charge in connection with expected sale														5,800,000
Additional cash income recognized due to distribution of refinancing proceeds											67,900,000
Additional income recognized on repayment of debt		971,000		971,000																							1,100,000
Operating partnership units issued (in shares)																															306,296
Committed additional contribution																															47,500,000
Committed additional capital contribution funded																															33,000,000
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	33,448,000	15,144,000	59,786,000	79,823,000																													38,700,000
Amount received from joint venture			26,099,000	150,893,000																													111,300,000
Floating rate financing assumed by joint venture			59,099,000	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%
Recognized gain (loss) on sale of interest in property	$ 9,534,000		$ 16,794,000							$ 3,000,000																														$ 7,300,000	$ 6,500,000	$ 4,000,000