Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) ft² building shares	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Real estate properties
Number of Properties | building	71
Approximate Square Feet	27,607,971
Weighted Average Occupancy unaudited (as a percent)	94.30%
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 1,580,306	$ 2,099,393	$ 2,114,041
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	61
Approximate Square Feet	25,499,092
Weighted Average Occupancy unaudited (as a percent)	94.20%
Residential
Real estate properties
Number of Properties | building	10
Approximate Square Feet	2,108,879
Weighted Average Occupancy unaudited (as a percent)	95.70%
Managed office properties
Real estate properties
Number of Properties | building	2
Approximate Square Feet	2,100,000
Debt and preferred equity investments including investments held by unconsolidated joint ventures | $	$ 1,700,000
Debt and preferred equity investments and other financing receivables included in other balance sheet items | $	$ 100,000
Consolidated properties
Real estate properties
Number of Properties | building	43
Approximate Square Feet	14,438,964
Consolidated properties | Commercial properties
Real estate properties
Number of Properties | building	41
Approximate Square Feet	13,993,859
Consolidated properties | Residential
Real estate properties
Number of Properties | building	2
Approximate Square Feet	445,105
Unconsolidated properties
Real estate properties
Number of Properties | building	28
Approximate Square Feet	13,169,007
Unconsolidated properties | Commercial properties
Real estate properties
Number of Properties | building	20
Approximate Square Feet	11,505,233
Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	8
Approximate Square Feet	1,663,774
Manhattan
Real estate properties
Number of Properties | building	52
Approximate Square Feet	24,402,292
Weighted Average Occupancy unaudited (as a percent)	94.60%
Manhattan | Office
Real estate properties
Number of Properties | building	30
Approximate Square Feet	23,603,274
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan | Retail
Real estate properties
Number of Properties | building	14
Approximate Square Feet	609,480
Weighted Average Occupancy unaudited (as a percent)	98.40%
Manhattan | Development/Redevelopment
Real estate properties
Number of Properties | building	7
Approximate Square Feet	189,538
Weighted Average Occupancy unaudited (as a percent)	84.90%
Manhattan | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Manhattan | Residential
Real estate properties
Number of Properties | building	10
Approximate Square Feet	2,108,879
Weighted Average Occupancy unaudited (as a percent)	95.70%
Manhattan | Consolidated properties
Real estate properties
Number of Properties | building	32
Approximate Square Feet	12,897,059
Manhattan | Consolidated properties | Office
Real estate properties
Number of Properties | building	20
Approximate Square Feet	12,387,091
Manhattan | Consolidated properties | Retail
Real estate properties
Number of Properties | building	6
Approximate Square Feet	320,430
Manhattan | Consolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	6
Approximate Square Feet	189,538
Manhattan | Consolidated properties | Fee Interest
Real estate properties
Number of Properties | building	0
Approximate Square Feet	0
Manhattan | Consolidated properties | Residential
Real estate properties
Number of Properties | building	2
Approximate Square Feet	445,105
Manhattan | Consolidated properties | Dual property type, retail portion
Real estate properties
Approximate Square Feet	270,132
Manhattan | Consolidated properties | Dual property type, residential portion
Real estate properties
Approximate Square Feet	222,855
Manhattan | Unconsolidated properties
Real estate properties
Number of Properties | building	20
Approximate Square Feet	11,505,233
Manhattan | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	10
Approximate Square Feet	11,216,183
Manhattan | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	8
Approximate Square Feet	289,050
Manhattan | Unconsolidated properties | Development/Redevelopment
Real estate properties
Number of Properties | building	1
Approximate Square Feet	0
Manhattan | Unconsolidated properties | Fee Interest
Real estate properties
Number of Properties | building	1
Approximate Square Feet	0
Manhattan | Unconsolidated properties | Residential
Real estate properties
Number of Properties | building	8
Approximate Square Feet	1,663,774
Suburban
Real estate properties
Number of Properties | building	9
Approximate Square Feet	1,096,800
Weighted Average Occupancy unaudited (as a percent)	86.40%
Suburban | Office
Real estate properties
Number of Properties | building	8
Approximate Square Feet	1,044,800
Weighted Average Occupancy unaudited (as a percent)	85.70%
Suburban | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet	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	1,096,800
Suburban | Consolidated properties | Office
Real estate properties
Number of Properties | building	8
Approximate Square Feet	1,044,800
Suburban | Consolidated properties | Retail
Real estate properties
Number of Properties | building	1
Approximate Square Feet	52,000
Suburban | Unconsolidated properties
Real estate properties
Number of Properties | building	0
Approximate Square Feet	0
Suburban | Unconsolidated properties | Office
Real estate properties
Number of Properties | building	0
Approximate Square Feet	0
Suburban | Unconsolidated properties | Retail
Real estate properties
Number of Properties | building	0
Approximate Square Feet	0