Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 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									1 Months Ended			1 Months Ended			1 Months Ended						3 Months Ended
	Mar. 31, 2013 item sqft	Mar. 31, 2012	Dec. 31, 2012	Apr. 30, 2012	Sep. 30, 2012 Mezzanine loans West Coast office portfolio	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	Jul. 31, 2012 One Court Square	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	May 31, 2011 280 Park Avenue	Mar. 31, 2011 280 Park Avenue	Mar. 31, 2013 280 Park Avenue sqft	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 sqft item	Apr. 30, 2012 379 West Broadway	Mar. 31, 2012 141 Fifth Avenue	Mar. 31, 2013 1552 Broadway sqft	Feb. 28, 2013 West Coast office portfolio	Sep. 30, 2012 West Coast office portfolio item sqft	Mar. 31, 2013 West Coast office portfolio	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 Two residential units item	Mar. 31, 2013 21 East 66th Street Three retail units	Mar. 31, 2013 21 East 66th Street Four residential units item	Mar. 31, 2013 315 West 36th Street sqft	Mar. 31, 2013 Herald Center sqft
Investments 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	$ 124,500,000		$ 117,700,000
General information on each joint venture
Ownership Interest (as a percent)				32.28%		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%
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%	40.00%	32.28%		32.28%		35.50%
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			13,045		4,500,000			460,000		17,000				148,000	365,000
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	50,000,000
Beneficial interest sold (as a percent)										50.00%
Sales price										617,600,000						481,100,000						2,800,000					143,600,000												48,500,000	46,000,000					315,000,000
Gain recognized on sale of beneficial interest																											0
Additional cash income recognized due to the distribution of refinancing proceeds										67,900,000
Recognized gain (loss) on sale of interest in property		7,260,000								3,000,000						1,000,000																							6,500,000	7,300,000					19,400,000
Triple-net lease arrangement																					13 years
Number of floors of student housing																																						30
Ownership sold (as a percent)																																										20.78%			49.50%
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																															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
Floating rate financing assumed by joint venture		59,099,000														315,000,000															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%																					16.14%
Number of properties	67																																										31						2
Number of properties																																																		3	4
Prior to the recapitalization positions held in mezzanine and preferred equity																																											26,700,000
Debt issued					68,000,000								300,000,000	290,000,000																													678,800,000
Term					2 years																																								7 years
Period of extension option for mortgage secured by the portfolio																																											1 year
Existing loan repaid												245,000,000																																	150,000,000
Interest rate, basis points (as a percent)												2.75%																																	2.20%
Interest rate, description												LIBOR																																	LIBOR
Ownership interest acquired (as a percent)																																										10.73%
Fees position acquired																																							$ 13,500,000
Interest rate, fixed rate debt (as a percent)																																															3.60%						8.75%