Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Billions	6 Months Ended
	Jun. 30, 2019 USD ($) ft² building shares
Real estate properties
Number of Properties | building	77
Approximate Square Feet unaudited (sqft)	29,540,006
Weighted Average Occupancy unaudited (as a percent)	93.20%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 2.3
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	67
Approximate Square Feet unaudited (sqft)	27,431,127
Weighted Average Occupancy unaudited (as a percent)	93.10%
Residential
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	2,108,879
Weighted Average Occupancy unaudited (as a percent)	94.50%
Managed office properties
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	2,100,000
Consolidated properties
Real estate properties
Number of Properties | building	50
Approximate Square Feet unaudited (sqft)	16,376,217
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	48
Approximate Square Feet unaudited (sqft)	15,931,112
Consolidated properties | Residential
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	445,105
Unconsolidated properties
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	13,163,789
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	11,500,015
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft)	1,663,774
Manhattan
Real estate properties
Number of Properties | building	52
Approximate Square Feet unaudited (sqft)	25,082,927
Weighted Average Occupancy unaudited (as a percent)	93.40%
Manhattan | Office
Real estate properties
Number of Properties | building	30
Approximate Square Feet unaudited (sqft)	23,603,274
Weighted Average Occupancy unaudited (as a percent)	93.80%
Manhattan | Retail
Real estate properties
Number of Properties | building	14
Approximate Square Feet unaudited (sqft)	609,480
Weighted Average Occupancy unaudited (as a percent)	98.90%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	870,173
Weighted Average Occupancy unaudited (as a percent)	78.60%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	2,108,879
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	33
Approximate Square Feet unaudited (sqft)	13,582,912
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	20
Approximate Square Feet unaudited (sqft)	12,387,091
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	325,648
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	6
Approximate Square Feet unaudited (sqft)	870,173
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	445,105
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	19
Approximate Square Feet unaudited (sqft)	11,500,015
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	11,216,183
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	283,832
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft)	0
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	8
Approximate Square Feet unaudited (sqft)	1,663,774
Suburban
Real estate properties
Number of Properties | building	15
Approximate Square Feet unaudited (sqft)	2,348,200
Weighted Average Occupancy unaudited (as a percent)	90.20%
Suburban | Office
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	2,295,200
Weighted Average Occupancy unaudited (as a percent)	90.00%
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	1
Approximate Square Feet unaudited (sqft)	1,000
Weighted Average Occupancy unaudited (as a percent)	0.00%
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	15
Approximate Square Feet unaudited (sqft)	2,348,200
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	2,295,200
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	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0
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	0
Approximate Square Feet unaudited (sqft)	0
Suburban | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft)	0