ACQUISITIONS, DEVELOPMENTS PLACED IN SERVICE AND DISPOSITIONS, PROPERTY DISPOSITIONS (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Jan. 31, 2015	Jan. 31, 2014
Dispositions [Abstract]
Sales price	$ 27,619	$ 80,883
Book value and sales cost	28,412	73,884
Gain/(loss)	(793)	6,999
Multi-Family Residential [Member]
Dispositions [Abstract]
Sales price	4,451	3,510
Book value and sales cost	3,033	3,738
Gain/(loss)	1,418	(228)
Number of properties sold		2
Multi-Family Residential [Member] | East Park Sioux Falls SD [Member]
Dispositions [Abstract]
Date disposed	Dec. 18, 2013
Sales price		2,214
Book value and sales cost		2,358
Gain/(loss)		(144)
Number of apartment units dispositions (in units)		84
Multi-Family Residential [Member] | Lancaster St Cloud MN [Member]
Dispositions [Abstract]
Date disposed	Sep. 22, 2014
Sales price	4,451
Book value and sales cost	3,033
Gain/(loss)	1,418
Number of apartment units dispositions (in units)	83
Multi-Family Residential [Member] | Sycamore Village - Sioux Falls, SD [Member]
Dispositions [Abstract]
Date disposed	Dec. 18, 2013
Sales price		1,296
Book value and sales cost		1,380
Gain/(loss)		(84)
Number of apartment units dispositions (in units)		48
Commercial Office [Member]
Dispositions [Abstract]
Sales price	14,825	12,950
Book value and sales cost	14,700	14,504
Gain/(loss)	125	(1,554)
Number of properties sold		3
Commercial Office [Member] | Northgate I Maple Grove MN [Member]
Dispositions [Abstract]
Date disposed	Dec. 01, 2014
Sales price	7,200
Book value and sales cost	6,881
Gain/(loss)	319
Area of a real estate property (in square feet)	79,297
Commercial Office [Member] | Bloomington Business Plaza, Bloomington, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 12, 2013
Sales price		4,500
Book value and sales cost		7,339
Gain/(loss)		(2,839)
Area of a real estate property (in square feet)		121,669
Commercial Office [Member] | Nicollet VII, Burnsville, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 12, 2013
Sales price		7,290
Book value and sales cost		6,001
Gain/(loss)		1,289
Area of a real estate property (in square feet)		118,125
Commercial Office [Member] | Pillsbury Business Center, Bloomington, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 12, 2013
Sales price		1,160
Book value and sales cost		1,164
Gain/(loss)		(4)
Area of a real estate property (in square feet)		42,929
Commercial Office [Member] | Dewey Hill - Edina, MN [Member]
Dispositions [Abstract]
Date disposed	May 19, 2014
Sales price	3,100
Book value and sales cost	3,124
Gain/(loss)	(24)
Area of a real estate property (in square feet)	73,338
Commercial Office [Member] | Wirth Corporate Center Golden Valley MN [Member]
Dispositions [Abstract]
Date disposed	Aug. 29, 2014
Sales price	4,525
Book value and sales cost	4,695
Gain/(loss)	(170)
Area of a real estate property (in square feet)	74,568
Commercial Industrial [Member]
Dispositions [Abstract]
Sales price	3,600	61,138
Book value and sales cost	5,393	52,079
Gain/(loss)	(1,793)	9,059
Number of properties sold		12
Commercial Industrial [Member] | Eagan 2785 & 2795 - Eagan, MN [Member]
Dispositions [Abstract]
Date disposed	Jul. 15, 2014
Sales price	3,600
Book value and sales cost	5,393
Gain/(loss)	(1,793)
Area of a real estate property (in square feet)	198,600
Commercial Industrial [Member] | Bodycote Industrial Building Eden Prairie MN [Member]
Dispositions [Abstract]
Date disposed	May 13, 2013
Sales price		3,150
Book value and sales cost		1,375
Gain/(loss)		1,775
Area of a real estate property (in square feet)		41,880
Commercial Industrial [Member] | Fargo 1320 45th St N Fargo ND [Member]
Dispositions [Abstract]
Date disposed	May 13, 2013
Sales price		4,700
Book value and sales cost		4,100
Gain/(loss)		600
Area of a real estate property (in square feet)		42,244
Commercial Industrial [Member] | Metal Improvement Company New Brighton MN [Member]
Dispositions [Abstract]
Date disposed	May 13, 2013
Sales price		2,350
Book value and sales cost		1,949
Gain/(loss)		401
Area of a real estate property (in square feet)		49,620
Commercial Industrial [Member] | Roseville 2929 Long Lake Road - Roseville, MN [Member]
Dispositions [Abstract]
Date disposed	May 13, 2013
Sales price		9,275
Book value and sales cost		9,998
Gain/(loss)		(723)
Area of a real estate property (in square feet)		172,057
Commercial Industrial [Member] | Brooklyn Park 7401 Boone Avenue, Brooklyn Park, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 12, 2013
Sales price		12,800
Book value and sales cost		12,181
Gain/(loss)		619
Area of a real estate property (in square feet)		322,751
Commercial Industrial [Member] | Cedar Lake Business Center, St. Louis Park, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 12, 2013
Sales price		2,550
Book value and sales cost		2,607
Gain/(loss)		(57)
Area of a real estate property (in square feet)		50,400
Commercial Industrial [Member] | API Building, Duluth, Minnesota [Member]
Dispositions [Abstract]
Date disposed	Sep. 24, 2013
Sales price		2,553
Book value and sales cost		1,488
Gain/(loss)		1,065
Area of a real estate property (in square feet)		35,000
Commercial Industrial [Member] | Lighthouse Duluth MN [Member]
Dispositions [Abstract]
Date disposed	Oct. 08, 2013
Sales price		1,825
Book value and sales cost		1,547
Gain/(loss)		278
Area of a real estate property (in square feet)		59,292
Commercial Industrial [Member] | Dixon Avenue Industrial Park, Des Moines, Iowa [Member]
Dispositions [Abstract]
Date disposed	Oct. 31, 2013
Sales price		14,675
Book value and sales cost		10,328
Gain/(loss)		4,347
Area of a real estate property (in square feet)		606,006
Commercial Industrial [Member] | Winsted Industrial Building Winsted MN [Member]
Dispositions [Abstract]
Date disposed	Jan. 17, 2014
Sales price		725
Book value and sales cost		747
Gain/(loss)		(22)
Area of a real estate property (in square feet)		41,685
Commercial Industrial [Member] | Minnetonka 13600 County Road 62 Minnetonka MN [Member]
Dispositions [Abstract]
Date disposed	Jan. 30, 2014
Sales price		3,800
Book value and sales cost		3,084
Gain/(loss)		716
Area of a real estate property (in square feet)		69,984
Commercial Industrial [Member] | Clive 2075 NW 94th Street, Clive, Iowa [Member]
Dispositions [Abstract]
Date disposed	Jan. 30, 2014
Sales price		2,735
Book value and sales cost		2,675
Gain/(loss)		60
Area of a real estate property (in square feet)		42,510
Commercial Retail [Member]
Dispositions [Abstract]
Sales price	4,073	3,285
Book value and sales cost	4,616	3,563
Gain/(loss)	(543)	(278)
Commercial Retail [Member] | Fargo Express Center & SC Pad - Fargo, ND [Member]
Dispositions [Abstract]
Date disposed	Nov. 18, 2014
Sales price	2,843
Book value and sales cost	2,211
Gain/(loss)	632
Area of a real estate property (in square feet)	64,226
Commercial Retail [Member] | Weston Retail Weston WI [Member]
Dispositions [Abstract]
Date disposed	Jul. 28, 2014
Sales price	0
Book value and sales cost	1,176
Gain/(loss)	(1,176)
Area of a real estate property (in square feet)	25,644
Commercial Retail [Member] | Kalispell Retail - Kalispell, MT [Member]
Dispositions [Abstract]
Date disposed	Oct. 15, 2014
Sales price	1,230
Book value and sales cost	1,229
Gain/(loss)	1
Area of a real estate property (in square feet)	52,000
Commercial Retail [Member] | Eagan Community Eagan MN [Member]
Dispositions [Abstract]
Date disposed	May 14, 2013
Sales price		2,310
Book value and sales cost		2,420
Gain/(loss)		(110)
Area of a real estate property (in square feet)		23,187
Commercial Retail [Member] | Anoka Strip Center Anoka MN [Member]
Dispositions [Abstract]
Date disposed	Dec. 23, 2013
Sales price		325
Book value and sales cost		347
Gain/(loss)		(22)
Area of a real estate property (in square feet)		10,625
Commercial Retail [Member] | Burnsville 2 Strip Center - Burnsville, MN [Member]
Dispositions [Abstract]
Date disposed	Jan. 08, 2014
Sales price		650
Book value and sales cost		796
Gain/(loss)		(146)
Area of a real estate property (in square feet)		8,400
Unimproved Land [Member]
Dispositions [Abstract]
Sales price	670
Book value and sales cost	670
Gain/(loss)	0
Unimproved Land [Member] | Kalispell Unimproved - Kalispell, MT [Member]
Dispositions [Abstract]
Date disposed	Oct. 15, 2014
Sales price	670
Book value and sales cost	670
Gain/(loss)	$ 0