Investment Securities - Schedule of investments in equity securities (Details) - USD ($) $ in Thousands	May 31, 2023	May 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 35,494	$ 33,758
Noncumulative Preferred Stock | Series C Preferred Stock
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	25,750	25,520
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 9,744	$ 8,238