Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended							1 Months Ended		1 Months Ended									1 Months Ended					1 Months Ended			1 Months Ended									1 Months Ended			3 Months Ended	1 Months Ended								1 Months Ended		1 Months Ended
	Mar. 31, 2013 entity sqft property	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 100 Park Avenue sqft	Mar. 31, 2013 21 West 34th Street sqft	Mar. 31, 2013 1604-1610 Broadway sqft	Mar. 31, 2013 27-29 West 34th Street sqft	Jun. 30, 2012 717 Fifth Avenue	Mar. 31, 2013 717 Fifth Avenue sqft	Jun. 30, 2012 717 Fifth Avenue Floating rate mortgage loan with maturity date of September 2012	Mar. 31, 2013 717 Fifth Avenue Mortgage loan	Mar. 31, 2013 717 Fifth Avenue Mezzanine loans	Mar. 31, 2013 800 Third Avenue sqft	Mar. 31, 2013 1745 Broadway sqft	Mar. 31, 2013 1 and 2 Jericho Plaza sqft	Mar. 31, 2013 16 Court Street sqft	Mar. 31, 2013 The Meadows sqft	Mar. 31, 2013 388 and 390 Greenwich Street sqft	Aug. 31, 2011 180/182 Broadway and 63 Nassu Street	Mar. 31, 2013 180/182 Broadway and 63 Nassu Street sqft	Mar. 31, 2013 600 Lexington Avenue sqft	Mar. 31, 2013 11 West 34th Street sqft	Mar. 31, 2013 7 Renaissance sqft	Sep. 30, 2012 3 Columbus Circle	Apr. 30, 2011 3 Columbus Circle	Mar. 31, 2013 3 Columbus Circle sqft	Mar. 31, 2013 280 Park Avenue sqft	May 31, 2011 280 Park Avenue	Mar. 31, 2011 280 Park Avenue	Mar. 31, 2013 1552-1560 Broadway sqft	May 31, 2012 155 West 46th Street	Mar. 31, 2013 747 Madison Avenue sqft	Mar. 31, 2013 724 Fifth Avenue sqft	Mar. 31, 2013 10 East 53rd Street sqft	Mar. 31, 2013 33 Beekman floor sqft	Feb. 28, 2013 West Coast office portfolio	Sep. 30, 2012 West Coast office portfolio property sqft	Mar. 31, 2013 West Coast office portfolio sqft	Sep. 30, 2012 West Coast office portfolio Mezzanine loans	Nov. 30, 2012 521 Fifth Avenue	Mar. 31, 2013 521 Fifth Avenue sqft	Apr. 30, 2013 21 East 66th Street	Mar. 31, 2013 21 East 66th Street sqft	Mar. 31, 2013 21 East 66th Street Three retail units unit	Mar. 31, 2013 21 East 66th Street 2 residential units unit	Mar. 31, 2013 21 East 66th Street Four residential units unit	Mar. 31, 2013 315 West 36th Street sqft	Jul. 31, 2012 One Court Square	Mar. 31, 2013 Herald Center sqft	Mar. 31, 2012 141 Fifth Avenue	Apr. 30, 2012 379 West Broadway	Mar. 31, 2013 1552 Broadway sqft
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	$ 124,500,000		$ 117,700,000
Beneficial interest sold (as a percent)								50.00%
Consideration received for sale of beneficial interest																								143,600,000
Gain recognized on sale of beneficial interest																								0
Ownership Interest (as a percent)				49.90%	50.00%	90.00%	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%			36.01%			50.50%		32.28%	32.28%		16.14%	35.50%		40.00%
Economic Interest (as a percent)				49.90%	50.00%	90.00%	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%			36.01%			50.50%		32.28%				35.50%		40.00%
Square Feet	29,721,879			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		4,500,000	4,474,000			460,000		17,000				148,000		365,000			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		50,000,000
Fees position acquired																																																			13,500,000
Triple-net lease arrangement																		13 years
Number of floors of student housing																																			30
Partnership interest sold to a third party (as a percent)																																				20.78%				49.50%
Sales price								617,600,000											2,800,000																					315,000,000								481,100,000		46,000,000	48,500,000
Additional cash income recognized due to distribution of refinancing proceeds								67,900,000
Existing loan repaid										245,000,000
Interest rate added to base rate (as a percent)										2.75%																														2.20%
Interest rate, description										LIBOR																														LIBOR
Debt issued											300,000,000	290,000,000																									678,800,000		68,000,000
Ownership interest acquired (as a percent)																																				10.73%
Interest rate, fixed rate debt (as a percent)																																										3.60%							8.75%
Ground rent	10,990,000	8,806,000
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																													286,600,000
Additional income upon the contribution of debt investment to joint venture	52,708,000	26,338,000																											38,700,000
Amount received from joint venture	5,333,000	16,652,000																									111,300,000
Prior to the recapitalization positions held in mezzanine and preferred equity																																					26,700,000
Term																																							2 years	7 years
Refinanced mortgage loan																																								150,000,000
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		59,099,000																											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		$ 7,260,000						$ 3,000,000																																$ 19,400,000								$ 1,000,000		$ 7,300,000	$ 6,500,000
Number of Properties	67																																				31