Investment Securities - Fair Value and Gross Unrealized Losses for Investments in a Gross Loss Position (Details) - USD ($)		Nov. 30, 2019	May 31, 2019
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, Fair value, held-to-maturity		$ 36,723,000	$ 52,665,000
Less than 12 months, Gross unrealized losses, held-to-maturity		(41,000)	(206,000)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value		16,088,000	179,374,000
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(70,000)	(761,000)
Total Fair Value		52,811,000	232,039,000
Gross Unrealized Losses		(111,000)	(967,000)
Commercial paper
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, Fair value, held-to-maturity	[1]		2,688,000
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value			1,000
Total Fair Value	[1]		2,688,000
Corporate debt securities
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, Fair value, held-to-maturity		29,640,000	45,999,000
Less than 12 months, Gross unrealized losses, held-to-maturity		(36,000)	(198,000)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value		15,621,000	164,086,000
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		(70,000)	(714,000)
Total Fair Value		45,261,000	210,085,000
Gross Unrealized Losses		(106,000)	(912,000)
Non-agency
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, Fair value, held-to-maturity		1,998,000	1,996,000
Less than 12 months, Gross unrealized losses, held-to-maturity		(2,000)	(4,000)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value			1,448,000
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss			(3,000)
Total Fair Value		1,998,000	3,444,000
Gross Unrealized Losses		(2,000)	(7,000)
Other ABS
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position [Abstract]
Less than 12 months, Fair value, held-to-maturity	[2]	5,085,000	1,982,000
Less than 12 months, Gross unrealized losses, held-to-maturity	[2]	(3,000)	(4,000)
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Fair Value	[2]	467,000	13,840,000
Debt Securities, Held-to-maturity, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	[2]	0	(44,000)
Total Fair Value	[2]	5,552,000	15,822,000
Gross Unrealized Losses	[2]	$ (3,000)	$ (48,000)

[1] Unrealized losses on the commercial paper investments are less than $1,000.

[2] Consists primarily of securities backed by auto lease loans, equipment-backed loans, auto loans and credit card loans.