Organization and Basis of Presentation (Schedule of Commercial Office Properties)(Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) ft² building shares	Dec. 31, 2014 USD ($)
Real estate properties
Number of Properties | building	113
Approximate Square Feet unaudited (sqft)	35,581,060
Weighted Average Occupancy unaudited (as a percent)	90.10%
Debt and preferred equity investments | $	$ 1,670,020	$ 1,408,804
Number of shares to be received on redemption of one unit of limited partnership interests (shares) | shares	1
Commercial properties
Real estate properties
Number of Properties | building	91
Approximate Square Feet unaudited (sqft)	32,558,438
Weighted Average Occupancy unaudited (as a percent)	89.80%
Residential
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft)	3,022,622
Weighted Average Occupancy unaudited (as a percent)	94.20%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	74
Approximate Square Feet unaudited (sqft)	27,356,262
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	69
Approximate Square Feet unaudited (sqft)	26,527,064
Consolidated properties | Residential
Real estate properties
Number of Properties | building	5
Approximate Square Feet unaudited (sqft)	829,198
Unconsolidated properties
Real estate properties
Number of Properties | building	39
Approximate Square Feet unaudited (sqft)	8,224,798
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft)	6,031,374
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet unaudited (sqft)	2,193,424
Manhattan
Real estate properties
Number of Properties | building	59
Approximate Square Feet unaudited (sqft)	27,564,497
Weighted Average Occupancy unaudited (as a percent)	91.70%
Manhattan | Office
Real estate properties
Number of Properties | building	32
Approximate Square Feet unaudited (sqft)	24,028,587
Weighted Average Occupancy unaudited (as a percent)	94.20%
Manhattan | Retail
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft)	756,963
Weighted Average Occupancy unaudited (as a percent)	89.10%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	1,995,417
Weighted Average Occupancy unaudited (as a percent)	59.00%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	783,530
Weighted Average Occupancy unaudited (as a percent)	100.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	2,956,011
Weighted Average Occupancy unaudited (as a percent)	94.20%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	41
Approximate Square Feet unaudited (sqft)	22,238,764
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	21,003,606
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft)	408,993
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	42,635
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	783,530
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	762,587
Manhattan | Consolidated properties | Dual property type, retail portion
Real estate properties
Approximate Square Feet unaudited (sqft)	270,132
Manhattan | Consolidated properties | Dual property type, residential portion
Real estate properties
Approximate Square Feet unaudited (sqft)	222,855
Manhattan | Unconsolidated properties
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft)	5,325,733
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	5
Approximate Square Feet unaudited (sqft)	3,024,981
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft)	347,970
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	1,952,782
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet unaudited (sqft)	2,193,424
Suburban
Real estate properties
Number of Properties | building	32
Approximate Square Feet unaudited (sqft)	4,993,941
Weighted Average Occupancy unaudited (as a percent)	79.20%
Suburban | Office
Real estate properties
Number of Properties | building	29
Approximate Square Feet unaudited (sqft)	4,940,941
Weighted Average Occupancy unaudited (as a percent)	79.00%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	52,000
Weighted Average Occupancy unaudited (as a percent)	100.00%
Suburban | Development/Redevelopment
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	1,000
Weighted Average Occupancy unaudited (as a percent)	100.00%
Suburban | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	66,611
Weighted Average Occupancy unaudited (as a percent)	94.40%
Suburban | Consolidated properties
Real estate properties
Number of Properties | building	28
Approximate Square Feet unaudited (sqft)	4,288,300
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	26
Approximate Square Feet unaudited (sqft)	4,235,300
Suburban | Consolidated properties | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	52,000
Suburban | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	1,000
Suburban | Consolidated properties | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	66,611
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	705,641
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	705,641
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0