Organization and Basis of Presentation (Schedule of Commercial Office Properties)(Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) ft² building shares	Dec. 31, 2015 USD ($)
Real estate properties
Number of Properties | building	109
Approximate Square Feet unaudited (sqft)	32,922,082
Mortgage Loans on Real Estate, Commercial and Consumer, Net Including Debt Investments, Other | $	$ 2,000,000
Weighted Average Occupancy unaudited (as a percent)	90.60%
Debt and preferred equity investments | $	$ 1,640,412	$ 1,670,020
Debt Investments, Other | $	$ 300,000
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	88
Approximate Square Feet unaudited (sqft)	29,202,213
Weighted Average Occupancy unaudited (as a percent)	91.50%
Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	3,719,869
Weighted Average Occupancy unaudited (as a percent)	83.80%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	68
Approximate Square Feet unaudited (sqft)	21,330,769
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	65
Approximate Square Feet unaudited (sqft)	20,858,664
Consolidated properties | Residential
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	472,105
Unconsolidated properties
Real estate properties
Number of Properties | building	41
Approximate Square Feet unaudited (sqft)	11,591,313
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	23
Approximate Square Feet unaudited (sqft)	8,343,549
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft)	3,247,764
Manhattan
Real estate properties
Number of Properties | building	58
Approximate Square Feet unaudited (sqft)	24,395,413
Weighted Average Occupancy unaudited (as a percent)	93.20%
Manhattan | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet unaudited (sqft)	22,612,745
Weighted Average Occupancy unaudited (as a percent)	94.90%
Manhattan | Retail
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	766,063
Weighted Average Occupancy unaudited (as a percent)	85.60%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	6
Approximate Square Feet unaudited (sqft)	813,149
Weighted Average Occupancy unaudited (as a percent)	52.30%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	203,456
Weighted Average Occupancy unaudited (as a percent)	100.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	3,719,869
Weighted Average Occupancy unaudited (as a percent)	83.80%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	38
Approximate Square Feet unaudited (sqft)	16,691,864
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	24
Approximate Square Feet unaudited (sqft)	16,054,606
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	418,093
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	1
Approximate Square Feet unaudited (sqft)	176,530
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	472,105
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	20
Approximate Square Feet unaudited (sqft)	7,703,549
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	6,558,139
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	3
Approximate Square Feet unaudited (sqft)	770,514
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	26,926
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft)	3,247,764
Suburban
Real estate properties
Number of Properties | building	30
Approximate Square Feet unaudited (sqft)	4,806,800
Weighted Average Occupancy unaudited (as a percent)	82.80%
Suburban | Office
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	4,753,800
Weighted Average Occupancy unaudited (as a percent)	82.60%
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	0
Approximate Square Feet unaudited (sqft)	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Suburban | Consolidated properties
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	4,166,800
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	25
Approximate Square Feet unaudited (sqft)	4,113,800
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	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	640,000
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	640,000
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