Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Billions	3 Months Ended
	Mar. 31, 2018 USD ($) ft² building shares
Real estate properties
Number of Properties | building	95
Approximate Square Feet unaudited (sqft)	32,449,593
Weighted Average Occupancy unaudited (as a percent)	91.40%
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	82
Approximate Square Feet unaudited (sqft)	29,820,737
Weighted Average Occupancy unaudited (as a percent)	91.60%
Residential
Real estate properties
Number of Properties | building	13
Approximate Square Feet unaudited (sqft)	2,628,856
Weighted Average Occupancy unaudited (as a percent)	88.90%
Managed office properties
Real estate properties
Approximate Square Feet unaudited (sqft)	336,000
Consolidated properties
Real estate properties
Number of Properties | building	58
Approximate Square Feet unaudited (sqft)	16,723,494
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	55
Approximate Square Feet unaudited (sqft)	16,251,389
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	37
Approximate Square Feet unaudited (sqft)	15,726,099
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	27
Approximate Square Feet unaudited (sqft)	13,569,348
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	57
Approximate Square Feet unaudited (sqft)	26,114,537
Weighted Average Occupancy unaudited (as a percent)	92.80%
Manhattan | Office
Real estate properties
Number of Properties | building	31
Approximate Square Feet unaudited (sqft)	24,552,255
Weighted Average Occupancy unaudited (as a percent)	94.20%
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)	44.40%
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	13
Approximate Square Feet unaudited (sqft)	2,628,856
Weighted Average Occupancy unaudited (as a percent)	88.90%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	33
Approximate Square Feet unaudited (sqft)	13,185,189
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	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	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	24
Approximate Square Feet unaudited (sqft)	12,929,348
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	11
Approximate Square Feet unaudited (sqft)	12,165,164
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	25
Approximate Square Feet unaudited (sqft)	3,706,200
Weighted Average Occupancy unaudited (as a percent)	83.30%
Suburban | Office
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft)	3,653,200
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)	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	22
Approximate Square Feet unaudited (sqft)	3,066,200
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	20
Approximate Square Feet unaudited (sqft)	3,013,200
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	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