Investment Securities - Debt Securities Trading, at Fair Value (Details) - USD ($) $ in Thousands	Nov. 30, 2021	May 31, 2021
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 588,615	$ 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	13,979	12,365
Corporate debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	504,032	497,944
Commercial agency mortgage-backed securities (“MBS”)
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	8,183	8,683
U.S. state and municipality debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	21,341	11,840
Foreign government debt securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	992	999
Other asset-backed-securities
Debt and Equity Securities, FV-NI [Line Items]
Debt securities trading, at fair value	$ 40,088	$ 42,843