Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) ft² building shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Real estate properties
Number of Properties \| building	69
Approximate Square Feet \| ft²	29,500,737
Weighted Average Occupancy unaudited (as a percent)	91.20%
Debt and preferred equity investments including investments held by unconsolidated joint ventures \| $	$ 1,076,542	$ 1,580,306	$ 2,099,393
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	60
Approximate Square Feet \| ft²	27,754,713
Weighted Average Occupancy unaudited (as a percent)	92.10%
Managed office properties
Real estate properties
Number of Properties \| building	2
Approximate Square Feet \| ft²	2,100,000
Debt and preferred equity investments including investments held by unconsolidated joint ventures \| $	$ 1,100,000
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	38
Approximate Square Feet \| ft²	12,765,702
Consolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	37
Approximate Square Feet \| ft²	12,683,452
Unconsolidated properties
Real estate properties
Number of Properties \| building	31
Approximate Square Feet \| ft²	16,735,035
Unconsolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	23
Approximate Square Feet \| ft²	15,071,261
Manhattan
Real estate properties
Number of Properties \| building	53
Approximate Square Feet \| ft²	26,891,913
Weighted Average Occupancy unaudited (as a percent)	92.50%
Manhattan \| Office
Real estate properties
Number of Properties \| building	29
Approximate Square Feet \| ft²	22,522,528
Weighted Average Occupancy unaudited (as a percent)	92.40%
Manhattan \| Retail
Real estate properties
Number of Properties \| building	13
Approximate Square Feet \| ft²	346,185
Weighted Average Occupancy unaudited (as a percent)	94.20%
Manhattan \| Development/Redevelopment
Real estate properties
Number of Properties \| building	11
Approximate Square Feet \| ft²	4,023,200
Manhattan \| Residential
Real estate properties
Number of Properties \| building	9
Approximate Square Feet \| ft²	1,746,024
Weighted Average Occupancy unaudited (as a percent)	75.70%
Manhattan \| Consolidated properties
Real estate properties
Number of Properties \| building	30
Approximate Square Feet \| ft²	11,820,652
Manhattan \| Consolidated properties \| Office
Real estate properties
Number of Properties \| building	18
Approximate Square Feet \| ft²	10,681,045
Manhattan \| Consolidated properties \| Retail
Real estate properties
Number of Properties \| building	4
Approximate Square Feet \| ft²	44,189
Manhattan \| Consolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	8
Approximate Square Feet \| ft²	1,095,418
Manhattan \| Consolidated properties \| Residential
Real estate properties
Number of Properties \| building	1
Approximate Square Feet \| ft²	82,250
Manhattan \| Unconsolidated properties
Real estate properties
Number of Properties \| building	23
Approximate Square Feet \| ft²	15,071,261
Manhattan \| Unconsolidated properties \| Office
Real estate properties
Number of Properties \| building	11
Approximate Square Feet \| ft²	11,841,483
Manhattan \| Unconsolidated properties \| Retail
Real estate properties
Number of Properties \| building	9
Approximate Square Feet \| ft²	301,996
Manhattan \| Unconsolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	3
Approximate Square Feet \| ft²	2,927,782
Manhattan \| Unconsolidated properties \| Residential
Real estate properties
Number of Properties \| building	8
Approximate Square Feet \| ft²	1,663,774
Suburban \| Office
Real estate properties
Number of Properties \| building	7
Approximate Square Feet \| ft²	862,800
Weighted Average Occupancy unaudited (as a percent)	83.30%
Suburban \| Consolidated properties \| Office
Real estate properties
Number of Properties \| building	7
Approximate Square Feet \| ft²	862,800
Suburban \| Unconsolidated properties \| Office
Real estate properties
Number of Properties \| building	0
Approximate Square Feet \| ft²	0