BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Compensating Balances and Other Investments Lender Holdbacks (Details) - USD ($)	6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015
Compensating Balances [Line Items]
Compensating balance	$ 14,818,000
Decrease in lender holdbacks	1,925,000	$ 3,258,000
Increase in lender holdbacks for improvements	614,000	$ 578,000
First International Bank, Watford City, North Dakota [Member]
Compensating Balances [Line Items]
Compensating balance	7,758,000
Associated Bank, Green Bay, Wisconsin [Member]
Compensating Balances [Line Items]
Compensating balance	3,000,000
Private Bank [Member]
Compensating Balances [Line Items]
Compensating balance	2,000,000
Bremer Bank [Member]
Compensating Balances [Line Items]
Compensating balance	1,285,000
Dacotah Bank, Minot, North Dakota [Member]
Compensating Balances [Line Items]
Compensating balance	250,000
Peoples State Bank of Velva, North Dakota [Member]
Compensating Balances [Line Items]
Compensating balance	225,000
American National Bank, Omaha, Nebraska [Member]
Compensating Balances [Line Items]
Compensating balance	200,000
Commerce Bank [Member]
Compensating Balances [Line Items]
Compensating balance	$ 100,000