COMMITMENTS AND CONTINGENCIES (Details) (USD $)	3 Months Ended			6 Months Ended
	Oct. 31, 2014		Oct. 31, 2013	Oct. 31, 2014		Oct. 31, 2013	Apr. 30, 2014
Real Estate Properties [Line Items]
Total property cost subject to purchase options	$ 2,013,770,000			$ 2,013,770,000			$ 1,996,031,000
Total gross rental revenue from properties subject to purchase options	58,835,000		54,477,000	115,961,000		108,100,000
Redemption basis				one-for-one
Period to hold the beneficial interest, 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	119,800,000		194,100,000	119,800,000		194,100,000
Tenant Improvements	9,100,000			9,100,000
Commitment period for tenant improvements				12 months
Anticipated Total Cost	355,273,000			355,273,000
Project costs incurred to date	211,760,000	[1]		211,760,000	[1]
Construction interest capitalized	1,200,000		781,000	2,200,000		1,400,000
Subject to purchase options [Member]
Real Estate Properties [Line Items]
Number of properties subject to purchase options	15			15
Total property cost subject to purchase options	120,500,000			120,500,000
Total gross rental revenue from properties subject to purchase options				5,200,000
Subject to restrictions on taxable dispositions [Member]
Real Estate Properties [Line Items]
Number of properties subject to restrictions on taxable dispositions	97			97
Area of real estate property subject to restrictions on taxable dispositions	4,200,000			4,200,000
Number of apartment units placed in service	4,963			4,963
Real estate investment amount of properties (net of accumulated depreciation)	728,400,000			728,400,000
Commons at Southgate, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	233	[2]		233	[2]
Anticipated Total Cost	37,201,000	[2]		37,201,000	[2]
Project costs incurred to date	34,787,000	[1],[2]		34,787,000	[1],[2]
Estimated year for completion				3Q 2015	[2]
Interest in joint venture (in hundredths)	52.90%	[2]		52.90%	[2]
Commons at Southgate, Minot, North Dakota [Member] | Footnote (1) [Member]
Real Estate Properties [Line Items]
Project costs incurred to date	19,300,000	[2]		19,300,000	[2]
Bismarck, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units pending acquisition	119			119
Purchase Price of Pending Acquisition				15,000,000
Value of limited partnership units	700,000			700,000
Cash paid for acquisition	14,300,000			14,300,000
Cardinal Point, Grand Forks, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	251			251
Anticipated Total Cost	40,042,000			40,042,000
Project costs incurred to date	13,565,000	[1]		13,565,000	[1]
Estimated year for completion				2Q 2016
Cypress Court - St. Cloud, MN [Member]
Real Estate Properties [Line Items]
Number of apartment units	66	[3]		66	[3]
Anticipated Total Cost	7,028,000	[3]		7,028,000	[3]
Project costs incurred to date	5,777,000	[1],[3]		5,777,000	[1],[3]
Estimated year for completion				3Q 2015	[3]
Interest in joint venture (in hundredths)	86.10%	[3]		86.10%	[3]
Grand Forks, ND [Member]
Real Estate Properties [Line Items]
Number of apartment units pending acquisition	74			74
Purchase Price of Pending Acquisition				9,300,000
Value of limited partnership units	400,000			400,000
Cash paid for acquisition	8,900,000			8,900,000
Minot, ND [Member] | Property One [Member]
Real Estate Properties [Line Items]
Square footage pending acquisition	45,339			45,339
Purchase Price of Pending Acquisition				2,800,000
Minot, ND [Member] | Property Two [Member]
Real Estate Properties [Line Items]
Square footage pending acquisition	1,680			1,680
Purchase Price of Pending Acquisition				1,300,000
Arcata Apartments, Golden Valley, Minnesota [Member]
Real Estate Properties [Line Items]
Number of apartment units	165			165
Anticipated Total Cost	33,448,000			33,448,000
Project costs incurred to date	24,689,000	[1]		24,689,000	[1]
Estimated year for completion				3Q 2015
Chateau II, Minot, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	72			72
Anticipated Total Cost	14,711,000			14,711,000
Project costs incurred to date	8,500,000	[1]		8,500,000	[1]
Estimated year for completion				1Q 2016
Deer Ridge - Jamestown, ND [Member]
Real Estate Properties [Line Items]
Number of apartment units	163			163
Anticipated Total Cost	24,519,000			24,519,000
Project costs incurred to date	5,646,000	[1]		5,646,000	[1]
Estimated year for completion				FY2016 Q2
RED 20 - Minneapolis, MN [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	10,625	[4]		10,625	[4]
Number of apartment units	130	[4]		130	[4]
Anticipated Total Cost	29,462,000	[4]		29,462,000	[4]
Project costs incurred to date	27,424,000	[1],[4]		27,424,000	[1],[4]
Estimated year for completion				3Q 2015	[4]
Interest in joint venture (in hundredths)	58.60%	[4]		58.60%	[4]
RED 20 - Minneapolis, MN [Member] | Footnote (1) [Member]
Real Estate Properties [Line Items]
Project costs incurred to date	11,600,000	[4]		11,600,000	[4]
Minot Office Center - Minot, ND [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	7,963			7,963
Anticipated Total Cost	2,923,000			2,923,000
Project costs incurred to date	1,300,000	[1]		1,300,000	[1]
Estimated year for completion				1Q 2016
71 France Phase I - Edina, MN [Member]
Real Estate Properties [Line Items]
Number of apartment units	109	[5]		109	[5]
Anticipated Total Cost	27,458,000	[5]		27,458,000	[5]
Project costs incurred to date	9,649,000	[1],[5]		9,649,000	[1],[5]
Estimated year for completion				2Q 2016	[5]
71 France Phase I - Edina, MN [Member] | Footnote (6) [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	21,772	[5]		21,772	[5]
Number of apartment units	241	[5]		241	[5]
Anticipated Total Cost	69,900,000	[5]		69,900,000	[5]
Interest in joint venture (in hundredths)	52.60%			52.60%
PrairieCare Medical - Brooklyn Park, MN [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	72,588			72,588
Anticipated Total Cost	24,251,000			24,251,000
Project costs incurred to date	7,458,000	[1]		7,458,000	[1]
Estimated year for completion				FY2016 Q2
Renaissance Heights I Apartments, Williston, North Dakota [Member]
Real Estate Properties [Line Items]
Number of apartment units	288	[6]		288	[6]
Anticipated Total Cost	62,362,000	[6]		62,362,000	[6]
Project costs incurred to date	50,607,000	[1],[6]		50,607,000	[1],[6]
Estimated year for completion				1Q 2016
Renaissance Heights I Apartments, Williston, North Dakota [Member] | Footnote (1) [Member]
Real Estate Properties [Line Items]
Project costs incurred to date	34,500,000	[6]		34,500,000	[6]
Minneapolis, MN [Member]
Real Estate Properties [Line Items]
Sales price of pending disposition				7,100,000
Weston, WI [Member]
Real Estate Properties [Line Items]
Sales price of pending disposition				5,300,000
Edina 6565 France SMC III - Edina, MN [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	57,479			57,479
Anticipated Total Cost	34,665,000			34,665,000
Project costs incurred to date	10,840,000	[1]		10,840,000	[1]
Estimated year for completion				FY2016 Q2
Minot Wells Fargo Bank - Minot, ND [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	4,998			4,998
Anticipated Total Cost	3,288,000			3,288,000
Project costs incurred to date	2,781,000	[1]		2,781,000	[1]
Estimated year for completion				3Q 2015
Roseville 3075 Long Lake Rd - Roseville, MN [Member]
Real Estate Properties [Line Items]
Area of real estate property subject to restrictions on taxable dispositions	202,807			202,807
Anticipated Total Cost	13,915,000			13,915,000
Project costs incurred to date	7,347,000	[1]		7,347,000	[1]
Estimated year for completion				1Q 2016
Other [Member]
Real Estate Properties [Line Items]
Project costs incurred to date	$ 1,390,000	[1]		$ 1,390,000	[1]

[1] Includes costs related to development projects that are placed in service in phases (Commons at Southgate - $19.3 million, Red 20 - $11.6 million, Renaissance Heights I - $34.5 million).

[2] The Company is currently an approximately 52.9% partner in the joint venture entity constructing this project; the anticipated total cost amount given is the total cost to the joint venture entity.

[3] The Company is an approximately 86.1% partner in the joint venture entity constructing this project; the anticipated total cost amount given is the total cost to the joint venture entity.

[4] The Company is an approximately 58.6% partner in the joint venture entity constructing this project; the anticipated total cost amount given is the total cost to the joint venture entity.

[5] The project will be constructed in three phases, and at the conclusion of construction of the third phase, the Company will have an approximately 52.6% interest in the joint venture entity constructing the project. The anticipated total cost amount given in the table above is the total cost to the joint venture entity of the projects first phase. The expected total project cost for all three phases is approximately $71.9 million for a total of approximately 241 residential units and approximately 21,772 square feet of retail space.

[6] The Company is an approximately 70% partner in the joint venture entity constructing this project; the anticipated total cost amount given is the total cost to the joint venture entity.