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Investment Securities Additional Information (Details) - USD ($)
12 Months Ended
May 31, 2019
May 31, 2018
May 31, 2017
Jun. 12, 2019
Debt and Equity Securities [Line Items]        
Unrealized losses on equity securities $ 1,799,000 $ 0 $ 0  
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax $ 0 $ 3,222,000 $ (4,614,000)  
Average Contractual Maturity of Held-to-maturity Securities 3 years      
Held To Maturity Securities Expected Debt Maturities Weighted Average Yield 2.98% 2.91%    
Held-to-maturity security in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions 187      
Proceeds from Sale, Maturity and Collection of Investments $ 0 $ 0    
Debt and Equity Securities, Realized Gain (Loss) $ 0 $ 0    
Subsequent Event        
Debt and Equity Securities [Line Items]        
Preferred Stock, Redemption Price Per Share       $ 25.00
Preferred Stock | Subsequent Event        
Debt and Equity Securities [Line Items]        
Equity securities       $ 25,000,000