ACQUISITIONS AND DISPOSITIONS - Dispositions (Details) - Disposed of by Sale	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($) property	Jun. 30, 2018 USD ($) ft² property	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($) ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property		0
Sale price of land and property	$ 7,300,000
Sales Price	10,304,000	$ 36,000,000	$ 10,304,000	$ 36,000,000
Book Value and Sale Cost	9,846,000	19,825,000	9,846,000	19,825,000
Gain/(Loss)	$ 614,000		458,000	16,175,000
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	1
Commercial Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	1
Other | Minot 1400 31st Ave SW - Minot, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	$ 6,530,000		6,530,000
Book Value and Sale Cost	6,048,000		6,048,000
Gain/(Loss)			482,000
Other | 43,404 sq ft Garden View - St. Paul, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price		14,000,000		14,000,000
Book Value and Sale Cost		$ 6,191,000		6,191,000
Gain/(Loss)				$ 7,809,000
Area of real estate property | ft²		43,404		43,404
Other | 52,116 sq ft Ritchie Medical - St. Paul, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price		$ 16,500,000		$ 16,500,000
Book Value and Sale Cost		$ 10,419,000		10,419,000
Gain/(Loss)				$ 6,081,000
Area of real estate property | ft²		52,116		52,116
Other | 22,187 sq ft Bismarck 715 East Broadway - Bismarck, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price		$ 5,500,000		$ 5,500,000
Book Value and Sale Cost		$ 3,215,000		3,215,000
Gain/(Loss)				$ 2,285,000
Area of real estate property | ft²		22,187		22,187
Unimproved Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	3,774,000		3,774,000
Book Value and Sale Cost	3,798,000		3,798,000
Gain/(Loss)			(24,000)
Unimproved Land | Creekside Crossing - Bismarck, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	3,049,000		3,049,000
Book Value and Sale Cost	3,205,000		3,205,000
Gain/(Loss)			(156,000)
Unimproved Land | Minot 1525 24th Ave SW - Minot, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	725,000		725,000
Book Value and Sale Cost	$ 593,000		593,000
Gain/(Loss)			$ 132,000