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Investment Securities - Additional Information (Details) - USD ($)
3 Months Ended 6 Months Ended
Nov. 30, 2019
Nov. 30, 2018
Nov. 30, 2019
Nov. 30, 2018
Jun. 12, 2019
May 31, 2019
Debt and Equity Securities [Line Items]            
Equity Securities Unrealized Loss $ 1,000,000 $ 2,000,000   $ 2,000,000    
Equity Securities Unrealized Gain     $ 2,000,000      
Preferred Stock, Redemption Price Per Share         $ 25.00  
Investment Securities in Unrealized Loss Positions, Number of Positions 65   65      
Average Contractual Maturity of Held-to-maturity Securities     3 years      
Held To Maturity Securities Expected Debt Maturities Weighted Average Yield 2.90%   2.90%     2.98%
Equity Securities, FV-NI, Realized Gain (Loss)     $ 200,000      
Gain (Loss) on Sale of Debt Investments $ 0 $ 0 $ 0 $ 0    
Preferred Stock            
Debt and Equity Securities [Line Items]            
Equity Securities, FV-NI, Cost         $ 25,000,000