Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended		6 Months Ended							1 Months Ended		1 Months Ended	3 Months Ended			1 Months Ended				6 Months Ended					1 Months Ended		1 Months Ended								1 Months Ended			1 Months Ended
	Jun. 30, 2013 item sqft	Jun. 30, 2012	Jun. 30, 2013 item sqft	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 100 Park Avenue sqft	Jun. 30, 2013 21 West 34th Street sqft	Jun. 30, 2013 1604-1610 Broadway sqft	Jun. 30, 2013 27-29 West 34th Street sqft	Jun. 30, 2012 717 Fifth Avenue	Jun. 30, 2013 717 Fifth Avenue sqft	Jun. 30, 2012 717 Fifth Avenue Floating rate mortgage loan with maturity date of September 2012	Jun. 30, 2013 717 Fifth Avenue Mortgage loan	Jun. 30, 2013 717 Fifth Avenue Mezzanine loans	Jun. 30, 2013 800 Third Avenue sqft	Jul. 31, 2012 One Court Square	Jun. 30, 2013 1745 Broadway sqft	Jun. 30, 2013 1 and 2 Jericho Plaza sqft	Jun. 30, 2013 The Meadows sqft	Jun. 30, 2013 388 and 390 Greenwich Street sqft	Jun. 30, 2013 180/182 Broadway and 63 Nassu Street sqft	Jun. 30, 2013 600 Lexington Avenue sqft	Jun. 30, 2013 11 West 34th Street sqft	Jun. 30, 2013 7 Renaissance sqft	Sep. 30, 2012 3 Columbus Circle	Jun. 30, 2013 3 Columbus Circle sqft	May 31, 2011 280 Park Avenue	Jun. 30, 2013 280 Park Avenue sqft	Jun. 30, 2013 1552-1560 Broadway sqft	May 31, 2012 155 West 46th Street	Jun. 30, 2013 747 Madison Avenue sqft	Jun. 30, 2013 724 Fifth Avenue sqft	Jun. 30, 2013 10 East 53rd Street sqft	Jun. 30, 2013 33 Beekman item sqft	Apr. 30, 2012 379 West Broadway	Mar. 31, 2012 141 Fifth Avenue	Jun. 30, 2013 1552 Broadway sqft	Jun. 30, 2013 West Coast office portfolio item sqft	Feb. 28, 2013 West Coast office portfolio	Sep. 30, 2012 West Coast office portfolio sqft item	Jun. 30, 2013 West Coast office portfolio Mortgage loan	Jun. 30, 2013 West Coast office portfolio Mezzanine loans	Sep. 30, 2012 West Coast office portfolio Mezzanine loans	Nov. 30, 2012 521 Fifth Avenue	Jun. 30, 2013 521 Fifth Avenue sqft	Jun. 30, 2013 21 East 66th Street sqft	Jun. 30, 2013 21 East 66th Street Two residential units item	Jun. 30, 2013 21 East 66th Street Three retail units item	Jun. 30, 2013 21 East 66th Street Four residential units item	Jun. 30, 2013 315 West 36th Street sqft	Jun. 30, 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	$ 133,400,000		$ 133,400,000		$ 117,700,000
General information on each joint venture
Ownership Interest (as a percent)						49.90%	50.00%	70.00%	50.00%		10.92%				42.95%		32.26%	20.26%	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%	70.00%	50.00%		10.92%				42.95%		32.26%	20.26%	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%
Square feet	29,721,879		29,721,879			834,000	30,000	30,000	41,000		120,000				526,000		674,000	640,000	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	4,067,000		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	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%
Obligation to fund additional contribution																										47,500,000
Sales price										617,600,000						481,100,000									143,600,000										48,500,000	46,000,000		111,900,000						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	(3,583,000)	9,534,000	(3,583,000)	16,794,000						3,000,000						1,000,000																			6,500,000	7,300,000		1,600,000						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%
Additional income upon the contribution of debt investment to joint venture	46,731,000	33,448,000	99,439,000	59,786,000
Amount received from joint venture			11,117,000	44,172,000
Floating rate financing assumed by joint venture				59,099,000												315,000,000
Effective ownership interest in underlying investment (as a percent)																											49.50%
Number of properties	67		67																																					31							2
Number of properties																																																3	4
Prior to the recapitalization positions held in mezzanine and preferred equity																																								26,700,000
Debt issued													300,000,000	290,000,000																										678,800,000			68,000,000	170,000,000
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%
Number of properties sold																																						1
Repayment of loan																																									99,000,000	7,300,000
Fees position acquired																																			$ 13,500,000
Interest rate, fixed rate debt (as a percent)																																																			8.75%