Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Billions	6 Months Ended
	Jun. 30, 2018 USD ($) ft² building shares
Real estate properties
Number of Properties | building	90
Approximate Square Feet unaudited (sqft)	31,150,063
Weighted Average Occupancy unaudited (as a percent)	91.90%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 2.4
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 0.2
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	77
Approximate Square Feet unaudited (sqft)	28,521,207
Weighted Average Occupancy unaudited (as a percent)	92.10%
Residential
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	2,628,856
Weighted Average Occupancy unaudited (as a percent)	90.00%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	57
Approximate Square Feet unaudited (sqft)	16,737,964
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	54
Approximate Square Feet unaudited (sqft)	16,265,859
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	33
Approximate Square Feet unaudited (sqft)	14,412,099
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	23
Approximate Square Feet unaudited (sqft)	12,255,348
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	56
Approximate Square Feet unaudited (sqft)	25,633,007
Weighted Average Occupancy unaudited (as a percent)	92.60%
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)	94.10%
Manhattan | Retail
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	650,553
Weighted Average Occupancy unaudited (as a percent)	94.20%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	11
Approximate Square Feet unaudited (sqft)	735,199
Weighted Average Occupancy unaudited (as a percent)	43.90%
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	13
Approximate Square Feet unaudited (sqft)	2,628,856
Weighted Average Occupancy unaudited (as a percent)	90.00%
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	4
Approximate Square Feet unaudited (sqft)	302,583
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft)	318,985
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	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	23
Approximate Square Feet unaudited (sqft)	12,255,348
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)	347,970
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	3
Approximate Square Feet unaudited (sqft)	416,214
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	21
Approximate Square Feet unaudited (sqft)	2,888,200
Weighted Average Occupancy unaudited (as a percent)	87.20%
Suburban | Office
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	2,835,200
Weighted Average Occupancy unaudited (as a percent)	87.00%
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	21
Approximate Square Feet unaudited (sqft)	2,888,200
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	19
Approximate Square Feet unaudited (sqft)	2,835,200
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