ACQUISITIONS AND DISPOSITIONS - Property Dispositions (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2018 USD ($) ft² property unit	Oct. 31, 2017 USD ($) ft² property unit	Oct. 31, 2018 USD ($) ft² unit	Oct. 31, 2017 USD ($) ft² property unit
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	$ 45,638	$ 41,866	$ 91,584	$ 82,844
Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price	$ 3,100	$ 63,400	52,185	66,862
Gain/(Loss)			8,992	$ 18,234
Commercial [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property	1
Parcels of Land [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property	1
Multi-Family Residential [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property		13
Sales Price			41,500
Gain/(Loss)			8,453
Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Dakota Commons - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			4,420
Gain/(Loss)			$ 542
Units \| unit	44		44
Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Williston Garden - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 12,310
Gain/(Loss)			$ 997
Interest in joint venture	74.11%		74.11%
Units \| unit	145		145
Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Renaissance Heights - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 24,770
Gain/(Loss)			$ 6,914
Interest in joint venture	87.14%		87.14%
Units \| unit	288		288
Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| 13 Multifamily - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property				13
Sales Price				$ 12,263
Gain/(Loss)				$ 701
Units \| unit		327		327
Healthcare [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property		3
Industrial [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property		1
Other [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 5,535	$ 51,424
Gain/(Loss)			587	17,546
Other [Member] \| Disposed of by Sale [Member] \| Minot Southgate Retail - Minot, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			1,925
Gain/(Loss)			$ (131)
Area of real estate property \| ft²	7,849		7,849
Other [Member] \| Disposed of by Sale [Member] \| Fresenius - Duluth, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 1,900
Gain/(Loss)			$ 822
Area of real estate property \| ft²	9,052		9,052
Other [Member] \| Disposed of by Sale [Member] \| Minot 2505 16Th St SW - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 1,710
Gain/(Loss)			$ (104)
Area of real estate property \| ft²	15,000		15,000
Other [Member] \| Disposed of by Sale [Member] \| Minot Southgate Wells Fargo Bank - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price				3,440
Gain/(Loss)				$ 108
Area of real estate property \| ft²		4,998		4,998
Other [Member] \| Disposed of by Sale [Member] \| Lexington Commerce Center - Eagan MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price				$ 9,000
Gain/(Loss)				$ 5,037
Area of real estate property \| ft²		90,260		90,260
Other [Member] \| Disposed of by Sale [Member] \| Duckwood Medical - Eagan MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price				$ 2,100
Gain/(Loss)				$ 214
Area of real estate property \| ft²		17,640		17,640
Other [Member] \| Disposed of by Sale [Member] \| Edgewood Vista - Hermantown, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price				$ 36,884
Gain/(Loss)				$ 12,187
Area of real estate property \| ft²		279,834		279,834
Unimproved Land [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of properties sold \| property		1
Sales Price			$ 5,150
Gain/(Loss)			(48)
Unimproved Land [Member] \| Disposed of by Sale [Member] \| Grand Forks - Grand Forks, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			3,000
Gain/(Loss)			14
Unimproved Land [Member] \| Disposed of by Sale [Member] \| Renaissance Heights - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			750
Gain/(Loss)			$ 66
Interest in joint venture	70.00%		70.00%
Unimproved Land [Member] \| Disposed of by Sale [Member] \| Badger Hills - Rochester MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price			$ 1,400
Gain/(Loss)			(128)
Unimproved Land [Member] \| Disposed of by Sale [Member] \| Bismarck 4916 - Bismarck, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sales Price				$ 3,175
Gain/(Loss)				(13)
Real Estate [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost	$ 19,336	$ 18,718	38,865	36,245
Real Estate [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			43,193	48,628
Real Estate [Member] \| Multi-Family Residential [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			33,047
Real Estate [Member] \| Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Dakota Commons - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			3,878
Real Estate [Member] \| Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Williston Garden - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			11,313
Real Estate [Member] \| Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| Renaissance Heights - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			17,856
Real Estate [Member] \| Multi-Family Residential [Member] \| Disposed of by Sale [Member] \| 13 Multifamily - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				11,562
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			4,948	33,878
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Minot Southgate Retail - Minot, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			2,056
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Fresenius - Duluth, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			1,078
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Minot 2505 16Th St SW - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			1,814
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Minot Southgate Wells Fargo Bank - Minot ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				3,332
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Lexington Commerce Center - Eagan MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				3,963
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Duckwood Medical - Eagan MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				1,886
Real Estate [Member] \| Other [Member] \| Disposed of by Sale [Member] \| Edgewood Vista - Hermantown, MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				24,697
Real Estate [Member] \| Unimproved Land [Member] \| Disposed of by Sale [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			5,198
Real Estate [Member] \| Unimproved Land [Member] \| Disposed of by Sale [Member] \| Grand Forks - Grand Forks, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			2,986
Real Estate [Member] \| Unimproved Land [Member] \| Disposed of by Sale [Member] \| Renaissance Heights - Williston, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			684
Real Estate [Member] \| Unimproved Land [Member] \| Disposed of by Sale [Member] \| Badger Hills - Rochester MN [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost			$ 1,528
Real Estate [Member] \| Unimproved Land [Member] \| Disposed of by Sale [Member] \| Bismarck 4916 - Bismarck, ND [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Book Value and Sale Cost				$ 3,188