Investment Securities - Schedule of composition of investments in debt securities (Details) - USD ($) $ in Thousands	May 31, 2023	May 31, 2022
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 474,875	$ 566,146
Commercial paper
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	0	9,985
Corporate debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	401,367	487,172
Commercial Agency MBS
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	7,237	7,815
U.S. state and municipality debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	27,300	27,778
Foreign government debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	974	967
Other ABS
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 37,997	$ 32,429