Leases - Additional Information (Details)		3 Months Ended		12 Months Ended
	Jan. 01, 2019 item	Sep. 30, 2020 USD ($) item	Jun. 30, 2020 item	Dec. 31, 2020 USD ($) item	Dec. 31, 2019 USD ($) item
Operating Leased Assets [Line Items]
Variable lease revenues				$ 57,700,000
Number of operating lease | item					9
Lease liability - operating lease				88,816,000	$ 56,762,000
Lease agreement purchase price				14,500
Operating lease right-of-use asset				76,268,000	$ 60,006,000
Number of finance lease | item					4
Finance lease, right-of-use asset				25,086,000	$ 102,055,000
565 Broadway | Core Portfolio
Operating Leased Assets [Line Items]
Operating lease right-of-use asset					4,900,000
Finance lease, right-of-use asset					19,400,000
110 University Place
Operating Leased Assets [Line Items]
Operating lease right-of-use asset					45,300,000
Operating lease liability					45,300,000
Capital Lease Liability
Operating Leased Assets [Line Items]
Finance lease liabilities					71,100,000
Fund IV Property
Operating Leased Assets [Line Items]
Impairment charge of right-of-use asset				$ 12,300,000
BSP II
Operating Leased Assets [Line Items]
Number of operating lease | item			1
Branch Plaza
Operating Leased Assets [Line Items]
Number of operating lease | item				1
Operating Lease term				22 years
1238 Wisconsin
Operating Leased Assets [Line Items]
Lease agreement purchase price				$ 11,500
Operating lease right-of-use asset				1,900
Operating lease liability				$ 1,900
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Operating lease right-of-use asset					11,900,000
Operating lease liability					12,800,000
Number of finance lease | item	3
Accounting Standards Update 2016-02 | 991 Madison Avenue Manhattan, NY
Operating Leased Assets [Line Items]
Capital lease term			49 years
Operating Lease term			15 years
Office Lease
Operating Leased Assets [Line Items]
Number of operating lease | item		1
Lease liability - operating lease		$ 1,700
Ground, Office and Equipment Leases
Operating Leased Assets [Line Items]
Operating lease right-of-use asset					82,600,000
Operating lease liability					76,600,000
Capital Leases Asset
Operating Leased Assets [Line Items]
Finance lease, right-of-use asset					$ 77,000,000.0
Minimum
Operating Leased Assets [Line Items]
Operating lease term				1 month
Maximum
Operating Leased Assets [Line Items]
Operating lease term				60 years