Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Billions	9 Months Ended
	Sep. 30, 2018 USD ($) ft² building shares
Real estate properties
Number of Properties | building	84
Approximate Square Feet unaudited (sqft)	30,584,045
Weighted Average Occupancy unaudited (as a percent)	93.00%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 2.1
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	70
Approximate Square Feet unaudited (sqft)	27,916,197
Weighted Average Occupancy unaudited (as a percent)	93.10%
Residential
Real estate properties
Number of Properties | building	14
Approximate Square Feet unaudited (sqft)	2,667,848
Weighted Average Occupancy unaudited (as a percent)	92.40%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	52
Approximate Square Feet unaudited (sqft)	16,236,956
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	48
Approximate Square Feet unaudited (sqft)	15,725,859
Consolidated properties | Residential
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft)	511,097
Unconsolidated properties
Real estate properties
Number of Properties | building	32
Approximate Square Feet unaudited (sqft)	14,347,089
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft)	12,190,338
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	2,156,751
Manhattan
Real estate properties
Number of Properties | building	55
Approximate Square Feet unaudited (sqft)	25,567,997
Weighted Average Occupancy unaudited (as a percent)	93.20%
Manhattan | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet unaudited (sqft)	24,247,255
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan | Retail
Real estate properties
Number of Properties | building	14
Approximate Square Feet unaudited (sqft)	664,797
Weighted Average Occupancy unaudited (as a percent)	98.40%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft)	655,945
Weighted Average Occupancy unaudited (as a percent)	40.10%
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	14
Approximate Square Feet unaudited (sqft)	2,667,848
Weighted Average Occupancy unaudited (as a percent)	92.40%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	33
Approximate Square Feet unaudited (sqft)	13,377,659
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	21
Approximate Square Feet unaudited (sqft)	12,756,091
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	5
Approximate Square Feet unaudited (sqft)	312,623
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet unaudited (sqft)	308,945
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	4
Approximate Square Feet unaudited (sqft)	511,097
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	22
Approximate Square Feet unaudited (sqft)	12,190,338
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft)	11,491,164
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft)	352,174
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft)	347,000
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	10
Approximate Square Feet unaudited (sqft)	2,156,751
Suburban
Real estate properties
Number of Properties | building	15
Approximate Square Feet unaudited (sqft)	2,348,200
Weighted Average Occupancy unaudited (as a percent)	91.70%
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)	91.60%
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	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