ACQUISITIONS AND DISPOSITIONS - Dispositions (Details) - Disposed of by Sale $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) ft² property	Mar. 31, 2018 USD ($) property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	$ 3,049	$ 36,000
Book value and sale cost	3,205	19,825
Gain/(Loss)	$ (156)	$ 16,175
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		3
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
Book value and sale cost	3,205
Gain/(Loss)	$ (156)
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
Book value and sale cost		6,191
Gain/(Loss)		7,809
Area of real estate property | ft²	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
Book value and sale cost		10,419
Gain/(Loss)		6,081
Area of real estate property | ft²	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
Book value and sale cost		3,215
Gain/(Loss)		$ 2,285
Area of real estate property | ft²	22,187