Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) ft² building shares
Real estate properties
Number of Properties | building	72
Approximate Square Feet unaudited (sqft) | ft²	30,328,709
Weighted Average Occupancy unaudited (as a percent)	91.90%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 1,150
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 100
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	63
Approximate Square Feet unaudited (sqft) | ft²	28,582,685
Weighted Average Occupancy unaudited (as a percent)	92.70%
Residential
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft) | ft²	1,746,024
Weighted Average Occupancy unaudited (as a percent)	78.80%
Managed office properties
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft) | ft²	2,100,000
Consolidated properties
Real estate properties
Number of Properties | building	42
Approximate Square Feet unaudited (sqft) | ft²	14,778,792
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	41
Approximate Square Feet unaudited (sqft) | ft²	14,696,542
Consolidated properties | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	82,250
Unconsolidated properties
Real estate properties
Number of Properties | building	30
Approximate Square Feet unaudited (sqft) | ft²	15,549,917
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft) | ft²	13,886,143
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft) | ft²	1,663,774
Manhattan
Real estate properties
Number of Properties | building	54
Approximate Square Feet unaudited (sqft) | ft²	27,485,885
Weighted Average Occupancy unaudited (as a percent)	93.10%
Manhattan | Office
Real estate properties
Number of Properties | building	29
Approximate Square Feet unaudited (sqft) | ft²	22,488,674
Weighted Average Occupancy unaudited (as a percent)	93.10%
Manhattan | Retail
Real estate properties
Number of Properties | building	12
Approximate Square Feet unaudited (sqft) | ft²	333,239
Weighted Average Occupancy unaudited (as a percent)	94.00%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	12
Approximate Square Feet unaudited (sqft) | ft²	4,663,972
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft) | ft²	1,746,024
Weighted Average Occupancy unaudited (as a percent)	78.80%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	32
Approximate Square Feet unaudited (sqft) | ft²	13,599,742
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	18
Approximate Square Feet unaudited (sqft) | ft²	10,647,191
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	4
Approximate Square Feet unaudited (sqft) | ft²	44,189
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	10
Approximate Square Feet unaudited (sqft) | ft²	2,908,362
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft) | ft²	0
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	82,250
Manhattan | Unconsolidated properties
Real estate properties
Number of Properties | building	22
Approximate Square Feet unaudited (sqft) | ft²	13,886,143
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	11
Approximate Square Feet unaudited (sqft) | ft²	11,841,483
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft) | ft²	289,050
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	2
Approximate Square Feet unaudited (sqft) | ft²	1,755,610
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft) | ft²	1,663,774
Suburban
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft) | ft²	1,096,800
Weighted Average Occupancy unaudited (as a percent)	85.70%
Suburban | Office
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft) | ft²	1,044,800
Weighted Average Occupancy unaudited (as a percent)	85.00%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	52,000
Weighted Average Occupancy unaudited (as a percent)	100.00%
Suburban | Consolidated properties
Real estate properties
Number of Properties | building	9
Approximate Square Feet unaudited (sqft) | ft²	1,096,800
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	8
Approximate Square Feet unaudited (sqft) | ft²	1,044,800
Suburban | Consolidated properties | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet unaudited (sqft) | ft²	52,000
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft) | ft²	0
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft) | ft²	0
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	0
Approximate Square Feet unaudited (sqft) | ft²	0