Subsequent Events (Details)					3 Months Ended								6 Months Ended
	Aug. 04, 2020 tenant $ / shares	Jul. 23, 2020 USD ($) ft² property	Jul. 09, 2020 USD ($)	Jul. 07, 2020 USD ($) ft² property	Jun. 30, 2020 USD ($) property	Jun. 30, 2020 USD ($) property $ / shares	Jun. 30, 2020 USD ($) property	Jun. 30, 2020 USD ($) ft² property	Jun. 30, 2020 USD ($) a property	Mar. 31, 2020 $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 $ / shares	Jun. 30, 2020 USD ($) property	Jun. 30, 2020 USD ($) property	Jun. 30, 2020 USD ($) ft² property	Jun. 30, 2020 USD ($) a property	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Subsequent Event [Line Items]
Mortgage loan maturing in 2021					$ 459,044,000	$ 459,044,000	$ 459,044,000	$ 459,044,000	$ 459,044,000				$ 459,044,000	$ 459,044,000	$ 459,044,000	$ 459,044,000
Credit facility, amount outstanding					0	0	0	0	0				0	0	0	0	$ 0
Cash and cash equivalents					$ 148,269,000	$ 148,269,000	$ 148,269,000	$ 148,269,000	$ 148,269,000		$ 117,188,000		$ 148,269,000	$ 148,269,000	$ 148,269,000	$ 148,269,000	$ 110,082,000	$ 31,004,000
Dividends per share, common stock (in dollars per share) | $ / shares						$ 0.27				$ 0.27	$ 0.24	$ 0.24
Improved land parcels
Subsequent Event [Line Items]
Number of properties | property					22	22	22	22	22				22	22	22	22
Asset Acquisitions 2020
Subsequent Event [Line Items]
Purchase price					$ 10,500,000								$ 41,100,000
Asset Acquisitions 2020 | Industrial building
Subsequent Event [Line Items]
Number of properties acquired | property							1							2
Area of real estate property acquired								13,000	2.8						79,000	5.5
Senior unsecured notes
Subsequent Event [Line Items]
Mortgage loan maturing in 2021					347,869,000	$ 347,869,000	$ 347,869,000	$ 347,869,000	$ 347,869,000				347,869,000	$ 347,869,000	$ 347,869,000	$ 347,869,000
Mortgage loans payable
Subsequent Event [Line Items]
Mortgage loan maturing in 2021					11,488,000	11,488,000	11,488,000	11,488,000	11,488,000				11,488,000	11,488,000	11,488,000	11,488,000
Secured loan					11,500,000	11,500,000	11,500,000	11,500,000	11,500,000				11,500,000	11,500,000	11,500,000	11,500,000
October 2022 unsecured revolving credit facility
Subsequent Event [Line Items]
Debt instrument, face amount					250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0				250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0
October 2022 unsecured revolving credit facility | Senior unsecured notes
Subsequent Event [Line Items]
Debt instrument, face amount					250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0				250,000,000.0	250,000,000.0	250,000,000.0	250,000,000.0
Credit facility | August 2020 unsecured revolving credit facility | Line of credit
Subsequent Event [Line Items]
Credit facility, amount outstanding					$ 0	$ 0	$ 0	$ 0	$ 0				$ 0	$ 0	$ 0	$ 0
Subsequent event
Subsequent Event [Line Items]
Lease termination fee			$ 3,300,000
Deferred rent receivable write-off			$ 3,400,000
Dividends per share, common stock (in dollars per share) | $ / shares	$ 0.29
Subsequent event | COVID-19
Subsequent Event [Line Items]
Percentage rents paid in cash	95.00%
Percentage rents paid by application of security deposit	1.00%
Number of tenants requesting deferral or abatement | tenant	174
Percent of tenant leases	36.00%
Percentage of annualized base rent	7.00%
Number of deferrals granted | tenant	61
Deferrals granted, percentage of annualized base rent	2.70%
Percentage granted by number	35.10%
Percentage granted by dollar amount	38.70%
Percentage of annualized base rent, total dollar deferral requests approved	3.60%
Percentage of total dollar requests	76.70%
Number of tenant's deferral requests denied | tenant	50
Tenant's request denied, percentage of annualized base rent	1.80%
Percentage of requests denied by number	28.70%
Percentage of requests denied by dollar amount	25.70%
Number of deferral and abatement requests rescinded | tenant	59
Percentage of annualized base rent	1.60%
Percentage of requests by number	33.30%
Percentage of requests by dollar amount	22.50%
Number of tenants in discussions | tenant	4
Percentage of annualized base rent	0.05%
Percentage by number	2.30%
Percentage by dollar amount	0.70%
Subsequent event | Industrial building | Third-party purchaser
Subsequent Event [Line Items]
Area of real estate property acquired | ft²		192,500
Number of properties | property		1
Sales price		$ 22,200,000
Subsequent event | Asset Acquisitions 2020 | Improved land parcels
Subsequent Event [Line Items]
Number of properties acquired | property				1
Area of real estate property acquired | ft²				22,000
Purchase price				$ 6,300,000