COMMITMENTS AND CONTINGENCIES (Details)	3 Months Ended
	Jul. 31, 2016 USD ($) ft² item property	Jul. 31, 2015 USD ($)	Apr. 30, 2016 USD ($)
Real Estate Properties [Line Items]
Property owned	$ 1,667,442,000		$ 1,681,471,000
Total gross rental revenue from properties subject to purchase options	$ 44,985,000	$ 40,750,000
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	$ 106,900,000	$ 100,300,000
Tenant improvements	$ 5,300,000
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
Commitment period to acquire joint venture interest	4 years
Subject to Purchase Options [Member]
Real Estate Properties [Line Items]
Number of properties | property	14
Property owned	$ 96,700,000
Total gross rental revenue from properties subject to purchase options	$ 2,200,000
Subject to Purchase Options [Member] | Spring Creek Senior Housing Properties [Member]
Real Estate Properties [Line Items]
Number of properties | property	8
Sales price	$ 43,500,000
Property owned	40,900,000
Total gross rental revenue from properties subject to purchase options	$ 881,000
Subject to Restrictions on Taxable Dispositions [Member]
Real Estate Properties [Line Items]
Number of properties | property	51
Area of real estate property | ft²	765,000
Number of apartment units | item	4,603
Real estate investment amount of properties (net of accumulated depreciation)	$ 436,400,000