COMMITMENTS AND CONTINGENCIES (Details) (USD $)	3 Months Ended
	Jul. 31, 2013 sqft	Jul. 31, 2012
Real Estate Properties [Line Items]
Number of properties subject to purchase options	14
Total property cost subject to purchase options	$ 92,800,000
Total gross rental revenue from properties subject to purchase options	2,000,000
Number of properties subject to restrictions on taxable dispositions	115
Area of a real estate property (in square feet)	6,400,000
Number of apartment units placed in service	4,936
Real estate investment amount of properties (net of accumulated depreciation)	876,100,000
Redemption basis	one-for-one
Redemption holding period, minimum	1 year
Number of consecutive trading days for valuation	10 days
Aggregate redemption value of UPREIT Units of operating partnership owned by limited partners	197,700,000	172,400,000
Joint ventures partner's interest acquired	0
Tenant Improvements	8,300,000
Commitment period for tenant improvements	12 months
Construction interest capitalized	580,000	161,000
Grand Forks Grand Forks, ND [Member]
Real Estate Properties [Line Items]
Number of apartment units	96
Purchase price of pending acquisition	10,600,000
Acquisition paid with issuance of equity	200,000
Bloomington Business Plaza, Bloomington, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	121,669
Sales price of property pending disposition	4,500,000
Brooklyn Park 7401 Boone Avenue, Brooklyn Park, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	322,751
Sales price of property pending disposition	12,800,000
Cedar Lake Business Center, St. Louis Park, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	50,400
Sales price of property pending disposition	2,600,000
Clive 2075 NW 94th Street, Clive, Iowa [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	42,510
Sales price of property pending disposition	2,700,000
Dixon Avenue Industrial Park, Des Moines, Iowa [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	606,006
Sales price of property pending disposition	14,700,000
Arcata Apartments, Golden Valley, Minnesota [Member]
Real Estate Properties [Line Items]
Area of land (in acres)	2
Number of apartment units	165
Land purchased	2,100,000
Estimated cost of construction	33,400,000
Project costs incurred to date	2,700,000
API Building, Duluth, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	35,000
Sales price of property pending disposition	2,600,000
Burnsville II Strip Center, Burnsville, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	8,400
Sales price of property pending disposition	650,000
Chateau II, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	32
Number of buildings	2
Estimated cost of construction	14,700,000
Project costs incurred to date	2,300,000
Dakota Commons, Williston, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	44
Land purchased	823,000
Estimated cost of construction	10,700,000
Project costs incurred to date	2,600,000
Eagan 2785 & 2795 Highway 55, Eagan, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	198,600
Sales price of property pending disposition	4,500,000
Nicollet VII, Burnsville, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	118,125
Sales price of property pending disposition	7,300,000
Pillsbury Business Center, Bloomington, Minnesota [Member]
Real Estate Properties [Line Items]
Pending Disposition (in square feet)	42,929
Sales price of property pending disposition	1,200,000
River Ridge Apartment Homes, Bismarck, ND [Member]
Real Estate Properties [Line Items]
Number of apartment units	146
Number of certificate of occupancy units	60
Estimated cost of construction	25,900,000
Project costs incurred to date	19,700,000
Cypress Court Apartment Homes, St. Cloud, Minnesota [Member]
Real Estate Properties [Line Items]
Number of apartment units	132
Number of buildings	2
Estimated cost of construction	14,300,000
Project costs incurred to date	10,100,000
Interest in joint venture (in hundredths)	79.00%
Interest in joint venture held by joint venture partner (in hundredths)	21.00%
Renaissance Heights I Apartments, Williston, North Dakota [Member]
Real Estate Properties [Line Items]
Area of land (in acres)	14.5
Acre of land purchased (in acres)	40
Number of apartment units	288
Number of buildings	5
Estimated cost of construction	62,400,000
Project costs incurred to date	16,900,000
Interest in joint venture (in hundredths)	70.00%
Southgate Apartments, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Interest in joint venture (in hundredths)	51.00%
Interest in joint venture held by joint venture partner (in hundredths)	49.00%
The Landing at Southgate Apartments, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	36
Number of buildings	3
Estimated cost of construction	15,000,000
Project costs incurred to date	12,500,000
The Commons at Southgate Apartments, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	233
Estimated cost of construction	37,200,000
Project costs incurred to date	$ 10,500,000