Real Estate Properties - Sale of Properties (Details) $ in Thousands										6 Months Ended
	Jun. 25, 2020 USD ($) ft² property	May 28, 2020 USD ($) ft² property	May 26, 2020 USD ($) ft² property	Mar. 28, 2020 USD ($) ft² property	Mar. 02, 2020 USD ($) ft² property	Feb. 14, 2020 USD ($) ft² property	Feb. 13, 2020 USD ($) ft² property	Feb. 06, 2020 USD ($) ft² property	Jan. 28, 2020 USD ($) ft² property	Jun. 30, 2020 USD ($) ft²	Jun. 30, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²										103,408
Gross Sales Price										$ 63,960	$ 308,200
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²										1,101,996
Gross Sales Price										$ 63,960
Disposed of by sale | Gothenburg, NE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property									1
Square Feet | ft²									31,978
Gross Sales Price									$ 585
Disposed of by sale | Rochester, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property								1
Square Feet | ft²								90,503
Gross Sales Price								$ 2,600
Disposed of by sale | Ainsworth, NE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property							1
Square Feet | ft²							32,901
Gross Sales Price							$ 775
Disposed of by sale | Dekalb, IL
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property						1
Square Feet | ft²						5,052
Gross Sales Price						$ 1,050
Disposed of by sale | Eau Claire, MI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property					1
Square Feet | ft²					98,824
Gross Sales Price					$ 2,600
Operating agreement annual rent and return					$ 817
Disposed of by sale | Stillwater, OK
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property				1
Square Feet | ft²				33,018
Gross Sales Price				$ 400
Disposed of by sale | Pawtucket, RI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property			1
Square Feet | ft²			22,027
Gross Sales Price			$ 1,610
Disposed of by sale | Canton, MA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		1
Square Feet | ft²		755,992
Gross Sales Price		$ 51,000
Operating agreement annual rent and return		$ 5,221
Disposed of by sale | Phoenix, AZ
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		1
Square Feet | ft²		29,434
Gross Sales Price		$ 2,900
Disposed of by sale | Bellefontaine, OH
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property	1
Square Feet | ft²	2,267
Gross Sales Price	$ 440