COMMITMENTS AND CONTINGENCIES (Details) $ in Thousands, ft² in Millions	3 Months Ended		9 Months Ended
	Jan. 31, 2016 USD ($) ft² Property Unit	Jan. 31, 2015 USD ($)	Jan. 31, 2016 USD ($) ft² Property Unit	Jan. 31, 2015 USD ($)	Apr. 30, 2015 USD ($)
Real Estate Properties [Line Items]
Total property cost subject to purchase options	$ 1,801,019		$ 1,801,019		$ 1,546,367
Total gross rental revenue from properties subject to purchase options	50,277	$ 46,753	$ 142,526	$ 135,621
Redemption basis			one-for-one
Number of consecutive trading days for valuation			10 days
Aggregate redemption value of UPREIT Units of operating partnership owned by limited partners	89,400	$ 122,000	$ 89,400	$ 122,000
Tenant Improvements	$ 4,500		$ 4,500
Commitment period for tenant improvements			12 months
Subject to Purchase Options [Member]
Real Estate Properties [Line Items]
Number of properties | Property	15		15
Total property cost subject to purchase options	$ 117,600		$ 117,600
Total gross rental revenue from properties subject to purchase options			6,800
Subject to Purchase Options [Member] | Nebraska Orthopaedic Hospital Omaha NE [Member]
Real Estate Properties [Line Items]
Total property cost subject to purchase options	$ 16,000		16,000
Total gross rental revenue from properties subject to purchase options			$ 1,300
Subject to Restrictions on Taxable Dispositions [Member]
Real Estate Properties [Line Items]
Number of properties | Property	78		78
Area of real estate property | ft²	2.7		2.7
Number of apartment units at properties subject to restrictions on taxable dispositions | Unit	5,372		5,372
Real estate investment amount of properties (net of accumulated depreciation)	$ 670,600		$ 670,600