Summary of Significant Accounting Policies (Detail)		3 Months Ended		9 Months Ended
	Jan. 01, 2019 USD ($)	Sep. 30, 2020 USD ($) property	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) property	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Feb. 25, 2014 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Restricted cash		$ 42,387,000	$ 0	$ 42,387,000	$ 0	$ 0	$ 0
Proceeds from sale of properties				$ 42,300,000
Escrow deposit								$ 1,400,000
Number of properties sold | property		3		3
Interest costs capitalized		$ 1,200,000	1,300,000	$ 3,106,000	2,993,000
Real estate taxes and insurance costs capitalized		400,000	400,000	$ 1,000,000.0	1,000,000.0
REIT annual taxable income distribution requirement percentage				90.00%
Income tax provision				$ 0	0
Adjustment to rental income resulting from lease collectability assessment		(1,500,000)	(300,000)	2,900,000	(800,000)
Cumulative effect of adoption of ASC 842					(222,000)
960-970 Knox Street
Summary Of Significant Accounting Policies [Line Items]
Escrow deposit		48,000		48,000
Construction Employees
Summary Of Significant Accounting Policies [Line Items]
Compensation costs capitalized		$ 1,000,000.0	$ 700,000	$ 3,000,000.0	$ 1,900,000
Minimum | Office Leases
Summary Of Significant Accounting Policies [Line Items]
Estimated remaining life				10 years
Minimum | Site Improvements
Summary Of Significant Accounting Policies [Line Items]
Estimated remaining life				5 years
Maximum | Office Leases
Summary Of Significant Accounting Policies [Line Items]
Estimated remaining life				30 years
Maximum | Site Improvements
Summary Of Significant Accounting Policies [Line Items]
Estimated remaining life				20 years
Measurement Input, Discount Rate | Minimum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		5.75%		5.75%
Measurement Input, Discount Rate | Maximum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		7.00%		7.00%
Measurement Input, Cap Rate | Minimum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		4.75%		4.75%
Measurement Input, Cap Rate | Maximum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		6.00%		6.00%
Property Average Lease Up Period | Minimum
Summary Of Significant Accounting Policies [Line Items]
Estimated average lease-up period				6 months
Property Average Lease Up Period | Maximum
Summary Of Significant Accounting Policies [Line Items]
Estimated average lease-up period				9 months
Estimate of Fair Value Measurement | Minimum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		3.00%		3.00%
Estimate of Fair Value Measurement | Maximum
Summary Of Significant Accounting Policies [Line Items]
Acquired property, measurement input		3.75%		3.75%
Accounting Standards Update 2016-02
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of adoption of ASC 842	$ 200,000