COMMITMENTS AND CONTINGENCIES (Details)	3 Months Ended
	Jul. 31, 2017 USD ($) ft² item property	Jul. 31, 2016 USD ($)	Jul. 01, 2017 USD ($)	Apr. 30, 2017 USD ($)
Real Estate Properties [Line Items]
Property owned	$ 1,744,252,000			$ 1,677,481,000
Total gross rental revenue from properties subject to purchase options	$ 47,647,000	$ 44,985,000
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	$ 91,500,000			$ 95,100,000
Tenant improvements	$ 4,700,000
Commitment period for tenant improvements	12 months
Loan commitment	$ 13,200,000		$ 16,200,000
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	2
Subject to Purchase Options [Member] | PrairieCare Medical - Brooklyn Park, MN [Member]
Real Estate Properties [Line Items]
Property owned	$ 24,500,000
Total gross rental revenue from properties subject to purchase options	621,000
Subject to Purchase Options [Member] | St Michael Clinic - St Michael, MN [Member]
Real Estate Properties [Line Items]
Property owned	2,900,000
Total gross rental revenue from properties subject to purchase options	$ 65,000
Subject to Restrictions on Taxable Dispositions [Member]
Real Estate Properties [Line Items]
Number of properties | property	31
Area of real estate property | ft²	596,000
Number of apartment units | item	3,476
Real estate investment amount of properties (net of accumulated depreciation)	$ 342,900,000