Investment Securities - Equity Securities (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 35,358	$ 35,102
Noncumulative Preferred Stock | Series C Preferred Stock
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	25,750	27,450
Common Stock
Debt and Equity Securities, FV-NI [Line Items]
Equity securities, at fair value	$ 9,608	$ 7,652