ACQUISITIONS AND DISPOSITIONS - Dispositions (Details) - Disposed of by Sale $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) property unit	Sep. 30, 2018 USD ($) ft² property unit	Sep. 30, 2019 USD ($) unit	Sep. 30, 2018 USD ($) ft² unit
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)	$ 38,900		$ 39,375	$ 25,271
Sale Price	95,329	$ 86,475	95,329	86,475
Book Value and Sale Cost	$ 55,954	$ 61,204	55,954	61,204
Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	1
Sale Price	$ 85,000		$ 85,000
Land | Renaissance Heights - Williston, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Apartment community, ownership percentage	70.00%		70.00%
Apartment Building
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property	6	3
Gain/(Loss)			$ 38,744	8,453
Sale Price	$ 84,425	$ 41,500	84,425	41,500
Book Value and Sale Cost	$ 45,681	$ 33,047	$ 45,681	$ 33,047
Apartment Building | 21 homes - Pinehurst - Billings, MT
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	21		21
Gain/(Loss)			$ 714
Sale Price	$ 1,675		1,675
Book Value and Sale Cost	$ 961		$ 961
Apartment Building | 160 homes - Brookfield Village - Topeka, KS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	160		160
Gain/(Loss)			$ 4,497
Sale Price	$ 10,350		10,350
Book Value and Sale Cost	$ 5,853		$ 5,853
Apartment Building | 220 homes - Crown Colony - Topeka, KS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	220		220
Gain/(Loss)			$ 9,324
Sale Price	$ 17,200		17,200
Book Value and Sale Cost	$ 7,876		$ 7,876
Apartment Building | 54 homes - Mariposa - Topeka, KS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	54		54
Gain/(Loss)			$ 1,810
Sale Price	$ 6,100		6,100
Book Value and Sale Cost	$ 4,290		$ 4,290
Apartment Building | 300 homes - Sherwood - Topeka, KS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	300		300
Gain/(Loss)			$ 14,614
Sale Price	$ 26,150		26,150
Book Value and Sale Cost	$ 11,536		$ 11,536
Apartment Building | 308 homes - Villa West - Topeka, KS
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit	308		308
Gain/(Loss)			$ 7,785
Sale Price	$ 22,950		22,950
Book Value and Sale Cost	$ 15,165		$ 15,165
Apartment Building | 44 homes - Dakota Commons - Williston, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit		44		44
Gain/(Loss)				$ 542
Sale Price		$ 4,420		4,420
Book Value and Sale Cost		$ 3,878		$ 3,878
Apartment Building | 145 homes - Williston Garden - Williston
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit		145		145
Gain/(Loss)				$ 997
Sale Price		$ 12,310		12,310
Book Value and Sale Cost		$ 11,313		$ 11,313
Apartment community, ownership percentage	74.11%		74.11%
Apartment Building | 288 homes - Renaissance Heights - Williston
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of apartment units | unit		288		288
Gain/(Loss)				$ 6,914
Sale Price		$ 24,770		24,770
Book Value and Sale Cost		$ 17,856		17,856
Apartment community, ownership percentage	87.14%		87.14%
Commercial Properties
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property		2
Other
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				16,866
Sale Price		$ 39,825		39,825
Book Value and Sale Cost		22,959		22,959
Other | Minot 1400 31st Ave SW - Minot, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)			$ 482
Sale Price	$ 6,530		6,530
Book Value and Sale Cost	6,048		6,048
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]
Gain/(Loss)				7,809
Sale Price		14,000		14,000
Book Value and Sale Cost		$ 6,191		$ 6,191
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]
Gain/(Loss)				$ 6,081
Sale Price		$ 16,500		16,500
Book Value and Sale Cost		$ 10,419		$ 10,419
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]
Gain/(Loss)				$ 2,285
Sale Price		$ 5,500		5,500
Book Value and Sale Cost		$ 3,215		$ 3,215
Area of real estate property | ft²		22,187		22,187
Other | 7,849 sq ft Minot Southgate Retail - Minot, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				$ (131)
Sale Price		$ 1,925		1,925
Book Value and Sale Cost		$ 2,056		$ 2,056
Area of real estate property | ft²		7,849		7,849
Other | 9,052 sq ft Fresenius - Duluth, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				$ 822
Sale Price		$ 1,900		1,900
Book Value and Sale Cost		$ 1,078		$ 1,078
Area of real estate property | ft²		9,052		9,052
Unimproved Land
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold | property		3
Gain/(Loss)			149	$ (48)
Sale Price	4,374	$ 5,150	4,374	5,150
Book Value and Sale Cost	4,225	5,198	4,225	5,198
Unimproved Land | Creekside Crossing - Bismarck, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)			(156)
Sale Price	3,049		3,049
Book Value and Sale Cost	3,205		3,205
Unimproved Land | Minot 1525 24th Ave SW - Minot, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)			132
Sale Price	725		725
Book Value and Sale Cost	593		593
Unimproved Land | Weston - Weston, WI
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)			173
Sale Price	600		600
Book Value and Sale Cost	$ 427		$ 427
Unimproved Land | Grand Forks - Grand Forks, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				14
Sale Price		3,000		3,000
Book Value and Sale Cost		2,986		2,986
Unimproved Land | Renaissance Heights - Williston, ND
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				66
Sale Price		750		750
Book Value and Sale Cost		684		684
Unimproved Land | Badger Hills Unimproved - Rochester, MN
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain/(Loss)				(128)
Sale Price		1,400		1,400
Book Value and Sale Cost		$ 1,528		$ 1,528