ACQUISITIONS, DEVELOPMENTS PLACED IN SERVICE, AND DISPOSITIONS - Acquisitions (Details) - Acquisitions [Member] $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2018 USD ($) ft² unit	Jan. 31, 2017 USD ($)	Jan. 31, 2018 USD ($) ft² unit
Acquisitions and development projects placed in service [Abstract]
Total acquisition costs	$ 90,600	$ 0	$ 244,350
Form of consideration, cash			244,350
Investment allocation, land	28,573		28,573
Investment allocation, building	212,870		212,870
Investment allocation, intangible assets	$ 2,907		$ 2,907
Multi-Family Residential [Member] | 191 unit - Oxbo - St. Paul, MN [Member]
Acquisitions and development projects placed in service [Abstract]
Number of apartment units | unit	191		191
Total acquisition costs			$ 61,500
Form of consideration, cash			61,500
Investment allocation, land	$ 5,809		5,809
Investment allocation, building	54,910		54,910
Investment allocation, intangible assets	$ 781		$ 781
Area of real estate property | ft²	11,477		11,477
Multi-Family Residential [Member] | 500 unit - Park Place - Plymouth, MN [Member]
Acquisitions and development projects placed in service [Abstract]
Number of apartment units | unit	500		500
Total acquisition costs			$ 92,250
Form of consideration, cash			92,250
Investment allocation, land	$ 10,609		10,609
Investment allocation, building	80,711		80,711
Investment allocation, intangible assets	$ 930		$ 930
Multi-Family Residential [Member] | 274 unit - Dylan - Denver Co [Member]
Acquisitions and development projects placed in service [Abstract]
Number of apartment units | unit	274		274
Total acquisition costs			$ 90,600
Form of consideration, cash			90,600
Investment allocation, land	$ 12,155		12,155
Investment allocation, building	77,249		77,249
Investment allocation, intangible assets	$ 1,196		$ 1,196