Investments in Unconsolidated Joint Ventures (Details) (USD $)	3 Months Ended		6 Months Ended							1 Months Ended		1 Months Ended	6 Months Ended											1 Months Ended		1 Months Ended								1 Months Ended			6 Months Ended		3 Months Ended	6 Months Ended	1 Months Ended							1 Months Ended			1 Months Ended
	Jun. 30, 2013 sqft property	Jun. 30, 2012	Jun. 30, 2013 entity sqft property	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	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 floor sqft	Jun. 30, 2013 West Coast office portfolio sqft	Feb. 28, 2013 West Coast office portfolio	Sep. 30, 2012 West Coast office portfolio sqft property	Jun. 30, 2013 West Coast office portfolio item sqft	Jun. 30, 2013 West Coast office portfolio Mortgage loan	Sep. 30, 2012 West Coast office portfolio Mezzanine loans	Jun. 30, 2013 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 Three retail units unit	Jun. 30, 2013 21 East 66th Street 2 residential units unit	Jun. 30, 2013 21 East 66th Street Four residential units unit	Jun. 30, 2013 315 West 36th Street sqft	Jul. 31, 2012 One Court Square	Jun. 30, 2013 Herald Center sqft	Mar. 31, 2013 Herald Center	Mar. 31, 2012 141 Fifth Avenue	Apr. 30, 2012 379 West Broadway	Jun. 30, 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	$ 133,400,000		$ 133,400,000		$ 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%	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%			36.01%					50.50%	32.28%	32.28%		16.14%	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%			36.01%					50.50%	32.28%				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	4,067,000		4,500,000	4,067,000					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	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			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																								111,900,000							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)																																																		8.75%
Number of properties sold																																					1
Repaid loan																																						99,000,000		7,300,000
Prior to the recapitalization positions held in mezzanine and preferred equity																																				26,700,000
Term																																							2 years		7 years
Amount of original debt being refinanced into another loan																																									150,000,000
Amount of new debt as result of refinancing																																									170,000,000
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																																												315,000,000
Effective ownership interest in underlying investment (as a percent)																										49.50%
Recognized gain (loss) on sale of interest in property	(3,583,000)	9,534,000	(3,583,000)	16,794,000						3,000,000																								1,600,000							19,400,000							1,000,000			7,300,000	6,500,000
Number of Properties	67		67																																	30
Committed additional capital contribution funded																									$ 47,500,000