Leases - Additional Information (Details) $ in Thousands	3 Months Ended		6 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020	Sep. 30, 2020 USD ($) item Aqreement Store	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) item
Operating Leased Assets [Line Items]
Variable lease revenues	$ 14,500	$ 14,700		$ 41,800	$ 41,200
Number of operating lease | item						9
Operating lease, right-of-use asset	89,615			89,615		$ 60,006
Lease liability - operating lease	89,437			89,437		$ 56,762
Number of finance lease | item						4
Finance lease, right-of-use asset	25,086			$ 25,086		$ 102,055
Pro rata core leased occupancy rate percentage			93.30%	91.10%
Pro rata fund leased occupancy rate percentage			91.10%	89.80%
Number of sores having bankruptcies, in core portfolio | Store				6
Core portfolio annualized base rents for stores of bankruptcies				$ 2,600
Core portfolio annualized base rents for stores of bankruptcies percentage				1.90%
Number of sores having bankruptcies, in fund portfolio | Store				17
Fund portfolio annualized base rents for stores of bankruptcies				$ 1,100
Fund portfolio annualized base rents for stores of bankruptcies percentage				5.20%
COVID-19
Operating Leased Assets [Line Items]
Number of rent concession agreements | Aqreement				237
Number of rent concession agreements for rent deferral | Aqreement				210
Number of rent concession agreements for rent abatements | Aqreement				27
Number of rent concession agreements accounted as no change in contract | Aqreement				203
Deferred receivables	7,500			$ 7,500
COVID-19 | Accounts Payable and Accrued Expenses
Operating Leased Assets [Line Items]
Deferred lease payments	$ 100			$ 100
565 Broadway | Core Portfolio
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset						4,900
Finance lease, right-of-use asset						19,400
110 University Place
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset						45,300
Operating lease liabilities						45,300
Capital Lease Liability
Operating Leased Assets [Line Items]
Finance lease liabilities						71,100
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			22 years
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset						11,900
Operating lease liabilities						$ 12,800
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			15 years
Office Lease
Operating Leased Assets [Line Items]
Number of operating lease | item				1
Operating lease, right-of-use asset	$ 1,700			$ 1,700
Lease liability - operating lease	$ 1,700			$ 1,700
Ground, Office and Equipment Leases
Operating Leased Assets [Line Items]
Operating lease, right-of-use asset						$ 82,600
Operating lease liabilities						76,600
Capital Leases Asset
Operating Leased Assets [Line Items]
Finance lease, right-of-use asset						$ 77,000
Minimum
Operating Leased Assets [Line Items]
Operating lease term	1 month			1 month
Maximum
Operating Leased Assets [Line Items]
Operating lease term	60 years			60 years