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BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES - Compensating Balances and Other Investments Lender Holdbacks (Details) - USD ($)
12 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Compensating Balances [Line Items]      
Compensating balance $ 13,160,000    
Decrease in lender holdbacks 4,347,000 $ 10,738,000 $ 3,780,000
Increase in lender holdbacks for improvements 1,136,000 $ 1,204,000 $ 11,045,000
First International Bank, Watford City, North Dakota [Member]      
Compensating Balances [Line Items]      
Compensating balance 6,000,000    
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 350,000    
Dacotah Bank, Minot, North Dakota [Member] | Other Investments [Member]      
Compensating Balances [Line Items]      
Certificates of deposit 50,000    
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