Real Estate Properties - Sale of Properties (Details) $ in Thousands													9 Months Ended
	Sep. 17, 2020 USD ($) ft² property	Aug. 26, 2020 USD ($) ft² property	Jul. 17, 2020 USD ($) ft² property	Jun. 25, 2020 USD ($) 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	Sep. 30, 2020 USD ($) ft²	Sep. 30, 2019 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet \| ft²													121,451
Gross Sales Price \| $													$ 67,811	$ 308,200
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Square Feet \| ft²													1,148,411
Gross Sales Price \| $													$ 69,835
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
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
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
Disposed of by sale \| Clinton, MD
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property			1
Square Feet \| ft²			2,935
Gross Sales Price \| $			$ 700
Disposed of by sale \| Lancaster, PA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property		1
Square Feet \| ft²		3,014
Gross Sales Price \| $		$ 775
Disposed of by sale \| Baton Rouge, LA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property		1
Square Feet \| ft²		2,334
Gross Sales Price \| $		$ 750
Disposed of by sale \| Winston Salem, NC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property		1
Square Feet \| ft²		32,816
Gross Sales Price \| $		$ 1,300
Disposed of by sale \| Hillard, OH
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Properties \| property	1
Square Feet \| ft²	5,316
Gross Sales Price \| $	$ 2,350