COMMITMENTS AND CONTINGENCIES (Details) (USD $)	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Real Estate Properties [Line Items]
Number of properties subject to purchase options	17		17
Total property cost subject to purchase options	$ 126,900,000		$ 126,900,000
Total gross rental revenue from properties subject to purchase options			7,900,000
Number of properties subject to restrictions on taxable dispositions	111		111
Real estate investment amount of properties (net of accumulated depreciation)	830,700,000		830,700,000
Redemption basis	one-for-one		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	198,900,000	144,700,000	198,900,000	144,700,000
Tenant Improvements	10,400,000		10,400,000
Commitment period for tenant improvements			12 months
Project costs incurred to date	125,000	466,000	125,000	466,000
Construction interest capitalized	157,000	116,000	438,000	230,000
Multi-Family Conversion, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Area of a real estate property (in square feet)	15,000		15,000
Number of apartment units	20		20
Estimated cost of construction	3,000,000		3,000,000
Project costs incurred to date	2,600,000		2,600,000
Senior Housing Memory Care and Assisted Living Units, Laramie, Wyoming [Member]
Real Estate Properties [Line Items]
Number of apartment units	48		48
Number of assisted living units	61		61
Number of memory care units	16		16
Number of assisted living units to be converted to memory care units	16		16
Number of additional assisted living units	29		29
Estimated cost of construction	3,800,000		3,800,000
Project costs incurred to date	3,600,000		3,600,000
Jamestown Medical Office Building, Jamestown, North Dakota [Member]
Real Estate Properties [Line Items]
Area of land (in acres)	4.9		4.9
Area of a real estate property (in square feet)	45,000		45,000
Estimated cost of construction	6,300,000		6,300,000
Interest in joint venture (in hundredths)	51.00%		51.00%
Cash contribution to project	1,500,000		1,500,000
Project costs incurred to date	9,200,000		9,200,000
Number of storey building	1		1
Lease term of property			79 years
Number of renewal of lease	2		2
Renewal of lease			10 years
Proceeds from construction loan			6,200,000
Branch Bank Building, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Area of a real estate property (in square feet)	3,700		3,700
Estimated cost of construction	1,100,000		1,100,000
Project costs incurred to date	1,700,000		1,700,000
Lease term of property			20 years
River Ridge Apartment Homes, Bismarck, ND [Member]
Real Estate Properties [Line Items]
Number of apartment units	146		146
Estimated cost of construction	24,200,000		24,200,000
Project costs incurred to date	5,900,000		5,900,000
Cypress Court Apartment Homes, St. Cloud, Minnesota [Member]
Real Estate Properties [Line Items]
Number of apartment units	132		132
Estimated cost of construction	14,300,000		14,300,000
Interest in joint venture (in hundredths)	79.00%		79.00%
Interest in joint venture held by joint venture partner (in hundredths)	21.00%		21.00%
Project costs incurred to date	4,500,000		4,500,000
Southgate Apartments, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	341		341
Interest in joint venture (in hundredths)	51.00%		51.00%
Interest in joint venture held by joint venture partner (in hundredths)	49.00%		49.00%
Project costs incurred to date	52,200,000		52,200,000
Minot IPS, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Area of land (in acres)	9.6		9.6
Area of a real estate property (in square feet)	28,000		28,000
Estimated cost of construction	$ 6,000,000		$ 6,000,000
Combined Commercial Segments [Member]
Real Estate Properties [Line Items]
Area of a real estate property (in square feet)	6,200,000		6,200,000
Multi Family Residential [Member]
Real Estate Properties [Line Items]
Number of apartment units	4,509		4,509