Real Estate Properties - Sale of Properties (Details) $ in Thousands							3 Months Ended
	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	Mar. 31, 2020 USD ($) ft²	Mar. 31, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²							821,068
Gross Sales Price							$ 8,010	$ 308,200
Gothenburg, NE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property						1
Rochester, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property					1
Ainsworth, NE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property				1
Dekalb, IL
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property			1
Eau Claire, MI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property		1
Stillwater, OK
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties | property	1
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²							292,276
Gross Sales Price							$ 8,010
Operating agreement annual rent and return		$ 817
Disposed of by sale | Gothenburg, NE
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
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]
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]
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]
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]
Square Feet | ft²		98,824
Gross Sales Price		$ 2,600
Disposed of by sale | Stillwater, OK
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet | ft²	33,018
Gross Sales Price	$ 400