Organization and Basis of Presentation (Schedule of Commercial Office Properties)(Details) $ in Billions	9 Months Ended
	Sep. 30, 2016 USD ($) ft² building shares
Real estate properties
Number of Properties | building	109
Approximate Square Feet unaudited (sqft)	32,918,879
Weighted Average Occupancy unaudited (as a percent)	90.30%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 1.8
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 0.3
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)	29,199,010
Weighted Average Occupancy unaudited (as a percent)	91.40%
Residential
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	3,719,869
Weighted Average Occupancy unaudited (as a percent)	81.40%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	69
Approximate Square Feet unaudited (sqft)	21,621,251
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	4
Approximate Square Feet unaudited (sqft)	762,587
Unconsolidated properties
Real estate properties
Number of Properties | building	40
Approximate Square Feet unaudited (sqft)	11,297,628
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	23
Approximate Square Feet unaudited (sqft)	8,340,346
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	17
Approximate Square Feet unaudited (sqft)	2,957,282
Manhattan
Real estate properties
Number of Properties | building	58
Approximate Square Feet unaudited (sqft)	24,392,210
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)	95.40%
Manhattan | Retail
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	766,063
Weighted Average Occupancy unaudited (as a percent)	87.60%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	6
Approximate Square Feet unaudited (sqft)	809,946
Weighted Average Occupancy unaudited (as a percent)	35.50%
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)	86.80%
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)	81.40%
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	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	20
Approximate Square Feet unaudited (sqft)	7,700,346
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)	767,311
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	17
Approximate Square Feet unaudited (sqft)	2,957,282
Suburban
Real estate properties
Number of Properties | building	30
Approximate Square Feet unaudited (sqft)	4,806,800
Weighted Average Occupancy unaudited (as a percent)	82.20%
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)	82.00%
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	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