Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Billions	9 Months Ended
	Sep. 30, 2017 USD ($) ft² building shares
Real estate properties
Number of Properties | building	99
Approximate Square Feet unaudited (sqft)	31,800,088
Weighted Average Occupancy unaudited (as a percent)	90.40%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 2.2
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 0.1
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	84
Approximate Square Feet unaudited (sqft)	28,671,127
Weighted Average Occupancy unaudited (as a percent)	90.80%
Residential
Real estate properties
Number of Properties | building	15
Approximate Square Feet unaudited (sqft)	3,128,961
Weighted Average Occupancy unaudited (as a percent)	86.40%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	63
Approximate Square Feet unaudited (sqft)	20,826,609
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	60
Approximate Square Feet unaudited (sqft)	20,354,504
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	36
Approximate Square Feet unaudited (sqft)	10,973,479
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	24
Approximate Square Feet unaudited (sqft)	8,316,623
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	12
Approximate Square Feet unaudited (sqft)	2,656,856
Manhattan
Real estate properties
Number of Properties | building	57
Approximate Square Feet unaudited (sqft)	24,369,327
Weighted Average Occupancy unaudited (as a percent)	92.20%
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)	93.40%
Manhattan | Retail
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	650,553
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	11
Approximate Square Feet unaudited (sqft)	929,499
Weighted Average Occupancy unaudited (as a percent)	59.90%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	176,530
Weighted Average Occupancy unaudited (as a percent)	100.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	15
Approximate Square Feet unaudited (sqft)	3,128,961
Weighted Average Occupancy unaudited (as a percent)	86.40%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	36
Approximate Square Feet unaudited (sqft)	16,692,704
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	4
Approximate Square Feet unaudited (sqft)	302,583
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	158,985
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	21
Approximate Square Feet unaudited (sqft)	7,676,623
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	4
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)	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	12
Approximate Square Feet unaudited (sqft)	2,656,856
Suburban
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	4,301,800
Weighted Average Occupancy unaudited (as a percent)	83.30%
Suburban | Office
Real estate properties
Number of Properties | building	24
Approximate Square Feet unaudited (sqft)	4,248,800
Weighted Average Occupancy unaudited (as a percent)	83.10%
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	24
Approximate Square Feet unaudited (sqft)	3,661,800
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft)	3,608,800
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