Organization and Basis of Presentation (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				3 Months Ended
	Mar. 31, 2013 sqft item	Dec. 31, 2012	Mar. 31, 2013 Stand-alone retail properties sqft item	Mar. 31, 2013 Development properties item sqft	Mar. 31, 2013 Residential properties item sqft	Mar. 31, 2013 Land interests sqft item	Mar. 31, 2013 Managed office properties item sqft	Mar. 31, 2013 Manhattan Consolidated properties sqft item	Mar. 31, 2013 Manhattan Unconsolidated properties sqft item	Mar. 31, 2013 Suburban Consolidated properties sqft item	Mar. 31, 2013 Suburban Unconsolidated properties item sqft	Sep. 30, 2012 West Coast office portfolio sqft item	Mar. 31, 2013 West Coast office portfolio Office properties sqft item
Investment in real estate properties
Number of Properties	67		14	15	3	2		27	9	26	5	31	31
Square feet	29,721,879		465,207	2,580,691	497,093	961,400	626,415	18,347,945	5,934,434	3,899,800	1,539,700	4,500,000	4,473,603
Weighted Average Occupancy (as a percent)	91.60%							93.80%	95.30%	78.40%	84.70%
Number of units					468
Number of office properties managed							2
Debt and preferred equity investments	$ 1,443,834	$ 1,357,203
Number of shares to be received on redemption of one unit of limited partnership interests	1