SUBSEQUENT EVENTS (Details) (USD $)	3 Months Ended		9 Months Ended
	Jan. 31, 2013	Jan. 31, 2012	Jan. 31, 2013	Jan. 31, 2012
Subsequent Event [Line Items]
Shares issued			$ 2,846,000	$ 3,413,000
Common stock, dividends, per share, declared (in dollars per share)	$ 0.1300	$ 0.1300	$ 0.3900	$ 0.4315
Sales price			10,173,000	2,175,000
Proceeds from sale of property			3,909,000	2,088,000
Omaha, Nebraska [Member]
Subsequent Event [Line Items]
Cash paid for acquisition of property	28,300,000		28,300,000
Rapid City, South Dakota [Member]
Subsequent Event [Line Items]
Cash paid for acquisition of property	2,800,000		2,800,000
Equity paid for acquisition of property	3,400,000		3,400,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Common stock, dividends, per share, declared (in dollars per share)	$ 13.00
Subsequent Event [Member] | Williston Apartments, Williston, North Dakota [Member]
Subsequent Event [Line Items]
Estimated cost of construction	62,400,000		62,400,000
Interest in joint venture (in hundredths)	70.00%		70.00%
Number of buildings	5		5
Area of land (in acres)	14.5		14.5
Area of vacant land acquired (in acres)	40		40
Number of property unit	288		288
Subsequent Event [Member] | Bismarck, North Dakota [Member]
Subsequent Event [Line Items]
Area of land (in acres)	18.2		18.2
Cash paid for acquisition of property	3,300,000		3,300,000
Subsequent Event [Member] | Vacant land in Minot, North Dakota [Member]
Subsequent Event [Line Items]
Area of land (in acres)	0.7		0.7
Cash paid for acquisition of property	172,000		172,000
Subsequent Event [Member] | Omaha, Nebraska [Member]
Subsequent Event [Line Items]
Number of buildings	9		9
Area of land (in acres)	18.5		18.5
Number of property unit	336		336
Subsequent Event [Member] | Rapid City, South Dakota [Member]
Subsequent Event [Line Items]
Purchase price of acquired property	6,200,000		6,200,000
Area of land (in acres)	3.5		3.5
Number of property unit	71		71
Subsequent Event [Member] | Vacant land in Grand Forks, North Dakota [Member]
Subsequent Event [Line Items]
Area of land (in acres)	10		10
Cash paid for acquisition of property	$ 1,600,000		$ 1,600,000