COMMITMENTS AND CONTINGENCIES (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2016 USD ($) ft² item property	Oct. 31, 2015 USD ($)	Oct. 31, 2016 USD ($) ft² item property	Oct. 31, 2015 USD ($)	Apr. 30, 2016 USD ($)
Real Estate Properties [Line Items]
Property owned	$ 1,665,354		$ 1,665,354		$ 1,681,471
Total gross rental revenue from properties subject to purchase options	45,859	$ 41,868	$ 90,844	$ 82,618
Redemption basis			one-for-one
Number of consecutive trading days for valuation			10 days
Aggregate redemption value of Units of operating partnership owned by limited partners	99,100	$ 112,600	$ 99,100	$ 112,600
Tenant improvements	$ 6,000		$ 6,000
Commitment period for tenant improvements			12 months
Southgate - Minot, ND [Member]
Real Estate Properties [Line Items]
Number of joint ventures with purchase commitment | item	1		1
Commitment period to acquire joint venture interest			4 years
Subject to Purchase Options [Member]
Real Estate Properties [Line Items]
Number of properties | property	7		7
Property owned	$ 77,400		$ 77,400
Total gross rental revenue from properties subject to purchase options			$ 3,700
Subject to Restrictions on Taxable Dispositions [Member]
Real Estate Properties [Line Items]
Number of properties | property	49		49
Area of real estate property | ft²	762,000		762,000
Number of apartment units | item	4,603		4,603
Real estate investment amount of properties (net of accumulated depreciation)	$ 435,900		$ 435,900