Investment Securities - Debt Securities Trading, at Fair Value (Details) - USD ($) $ in Thousands	Feb. 28, 2022	May 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 565,426	$ 576,175
Certificates of deposit
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	0	1,501
Commercial paper
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	13,969	12,365
Corporate debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	483,588	497,944
Commercial agency mortgage-backed securities (“MBS”)
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	7,786	8,683
U.S. state and municipality debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	21,396	11,840
Foreign government debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	979	999
Other asset-backed-securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 37,708	$ 42,843