Organization and Basis of Presentation (Details 2) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013 sqft property	Dec. 31, 2012
Real estate properties
Number of Properties	67
Square Feet	29,721,879
Weighted Average Occupancy (as a percent)	91.60%
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
Stand-alone retail properties
Real estate properties
Number of Properties	14
Square Feet	465,207
Development
Real estate properties
Number of Properties	15
Square Feet	2,580,691
Residential properties
Real estate properties
Number of Properties	3
Square Feet	497,093
Number of units	468
Land
Real estate properties
Number of Properties	2
Square Feet	961,400
Managed office properties
Real estate properties
Square Feet	626,415
Number of office properties managed	2
Manhattan | Consolidated properties
Real estate properties
Number of Properties	27
Square Feet	18,347,945
Weighted Average Occupancy (as a percent)	93.80%
Manhattan | Unconsolidated properties
Real estate properties
Number of Properties	9
Square Feet	5,934,434
Weighted Average Occupancy (as a percent)	95.30%
Suburban | Consolidated properties
Real estate properties
Number of Properties	26
Square Feet	3,899,800
Weighted Average Occupancy (as a percent)	78.40%
Suburban | Unconsolidated properties
Real estate properties
Number of Properties	5
Square Feet	1,539,700
Weighted Average Occupancy (as a percent)	84.70%
West Coast | Office properties
Real estate properties
Number of Properties	31
Square Feet	4,473,603