Investment Securities - Debt Securities and Corresponding Gross Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	Nov. 30, 2019	May 31, 2019
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	$ 573,547	$ 565,444
Gross Unrealized Gains	10,003	6,072
Gross Unrealized Losses	(111)	(967)
Debt Securities, Held-to-maturity, Fair Value	583,439	570,549
Certificates of deposit
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	8,687	1,000
Gross Unrealized Gains	1	0
Gross Unrealized Losses	0	0
Debt Securities, Held-to-maturity, Fair Value	8,688	1,000
Commercial paper
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	4,972	12,395
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Debt Securities, Held-to-maturity, Fair Value	4,972	12,395
U.S. treasury and agency debt securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	4,566
Gross Unrealized Gains	145
Gross Unrealized Losses	0
Debt Securities, Held-to-maturity, Fair Value	4,711
U.S. agency debt securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost		3,207
Gross Unrealized Gains		108
Gross Unrealized Losses		0
Debt Securities, Held-to-maturity, Fair Value		3,315
Corporate debt securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	483,791	478,578
Gross Unrealized Gains	8,620	4,989
Gross Unrealized Losses	(106)	(912)
Debt Securities, Held-to-maturity, Fair Value	492,305	482,655
Agency
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	7,235	7,255
Gross Unrealized Gains	384	291
Gross Unrealized Losses	0	0
Debt Securities, Held-to-maturity, Fair Value	7,619	7,546
Non-agency
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	3,327	3,453
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(2)	(7)
Debt Securities, Held-to-maturity, Fair Value	3,326	3,446
Total commercial MBS
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	10,562	10,708
Gross Unrealized Gains	385	291
Gross Unrealized Losses	(2)	(7)
Debt Securities, Held-to-maturity, Fair Value	10,945	10,992
U.S. state and municipality debt securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	9,604	9,608
Gross Unrealized Gains	458	352
Gross Unrealized Losses	0	0
Debt Securities, Held-to-maturity, Fair Value	10,062	9,960
Foreign government debt securities
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	1,260	1,254
Gross Unrealized Gains	50	42
Gross Unrealized Losses	0	0
Debt Securities, Held-to-maturity, Fair Value	1,310	1,296
Other ABS
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Debt securities held-to-maturity, at amortized cost	50,105	48,694
Gross Unrealized Gains	344	290
Gross Unrealized Losses	(3)	(48)
Debt Securities, Held-to-maturity, Fair Value	$ 50,446	$ 48,936