Organization and Basis of Presentation - Schedule of Commercial Office Properties (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) ft² building shares	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)
Real estate properties
Number of Properties \| building	81
Approximate Square Feet	30,177,253
Weighted Average Occupancy unaudited (as a percent)	92.90%
Investment in mortgage loans on real estate \| $	$ 2,099,393	$ 2,114,041	$ 1,640,412
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	69
Approximate Square Feet	27,575,397
Weighted Average Occupancy unaudited (as a percent)	93.10%
Residential
Real estate properties
Number of Properties \| building	12
Approximate Square Feet	2,601,856
Weighted Average Occupancy unaudited (as a percent)	91.50%
Managed office properties
Real estate properties
Number of Properties \| building	2
Approximate Square Feet	2,100,000
Investment in mortgage loans on real estate \| $	$ 2,100,000
Debt investments, other \| $	$ 100,000
Consolidated properties
Real estate properties
Number of Properties \| building	49
Approximate Square Feet	15,992,145
Consolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	47
Approximate Square Feet	15,547,040
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	32
Approximate Square Feet	14,185,108
Unconsolidated properties \| Commercial properties
Real estate properties
Number of Properties \| building	22
Approximate Square Feet	12,028,357
Unconsolidated properties \| Residential
Real estate properties
Number of Properties \| building	10
Approximate Square Feet	2,156,751
Manhattan
Real estate properties
Number of Properties \| building	54
Approximate Square Feet	25,227,197
Weighted Average Occupancy unaudited (as a percent)	93.20%
Manhattan \| Office
Real estate properties
Number of Properties \| building	30
Approximate Square Feet	23,716,274
Weighted Average Occupancy unaudited (as a percent)	94.50%
Manhattan \| Retail
Real estate properties
Number of Properties \| building	16
Approximate Square Feet	677,822
Weighted Average Occupancy unaudited (as a percent)	96.70%
Manhattan \| Development/Redevelopment
Real estate properties
Number of Properties \| building	7
Approximate Square Feet	833,101
Weighted Average Occupancy unaudited (as a percent)	54.10%
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	12
Approximate Square Feet	2,601,856
Weighted Average Occupancy unaudited (as a percent)	91.50%
Manhattan \| Consolidated properties
Real estate properties
Number of Properties \| building	32
Approximate Square Feet	13,198,840
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	7
Approximate Square Feet	325,648
Manhattan \| Consolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	5
Approximate Square Feet	486,101
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	22
Approximate Square Feet	12,028,357
Manhattan \| Unconsolidated properties \| Office
Real estate properties
Number of Properties \| building	10
Approximate Square Feet	11,329,183
Manhattan \| Unconsolidated properties \| Retail
Real estate properties
Number of Properties \| building	9
Approximate Square Feet	352,174
Manhattan \| Unconsolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	2
Approximate Square Feet	347,000
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	10
Approximate Square Feet	2,156,751
Suburban
Real estate properties
Number of Properties \| building	15
Approximate Square Feet	2,348,200
Weighted Average Occupancy unaudited (as a percent)	91.40%
Suburban \| Office
Real estate properties
Number of Properties \| building	13
Approximate Square Feet	2,295,200
Weighted Average Occupancy unaudited (as a percent)	91.30%
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 \| Development/Redevelopment
Real estate properties
Number of Properties \| building	1
Approximate Square Feet	1,000
Weighted Average Occupancy unaudited (as a percent)	0.00%
Suburban \| Residential
Real estate properties
Number of Properties \| building	0
Approximate Square Feet	0
Weighted Average Occupancy unaudited (as a percent)	0.00%
Suburban \| Consolidated properties
Real estate properties
Number of Properties \| building	15
Approximate Square Feet	2,348,200
Suburban \| Consolidated properties \| Office
Real estate properties
Number of Properties \| building	13
Approximate Square Feet	2,295,200
Suburban \| Consolidated properties \| Retail
Real estate properties
Number of Properties \| building	1
Approximate Square Feet	52,000
Suburban \| Consolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	1
Approximate Square Feet	1,000
Suburban \| Consolidated properties \| Residential
Real estate properties
Number of Properties \| building	0
Approximate Square Feet	0
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
Suburban \| Unconsolidated properties \| Development/Redevelopment
Real estate properties
Number of Properties \| building	0
Approximate Square Feet	0
Suburban \| Unconsolidated properties \| Residential
Real estate properties
Number of Properties \| building	0
Approximate Square Feet	0