Subordinated Deferrable Debt - Narrative (Details) $ in Thousands	12 Months Ended
	May 31, 2026 USD ($) period loan	May 31, 2025 USD ($)	May 31, 2024 USD ($)
Debt Instrument [Line Items]
Repayments of subordinated debt	$ 650,236	$ 0	$ 100,000
Consecutive quarterly periods for which interest payment can be deferred | period	40
Proceeds from issuance of subordinated deferrable debt	$ 629,381	$ 43,811	$ 103,500
Term in Years		30 years
Retail Investors in Capital Markets
Debt Instrument [Line Items]
Proceeds from issuance of subordinated deferrable debt	29,000
Variable rate issuance 2013
Debt Instrument [Line Items]
Repayments of subordinated debt	650,000
Total aggregate principal amount	$ 0	$ 300,000
Term in Years	30 years
Variable rate issuance 2013 | Debt Instrument, Redemption, Period Two
Debt Instrument [Line Items]
Loss on early extinguishment of debt	$ 6,000
Subordinated Deferrable Notes With Variable Interest Rates Due 2046
Debt Instrument [Line Items]
Total aggregate principal amount		350,000
5.50% issuance 2019
Debt Instrument [Line Items]
Total aggregate principal amount	$ 250,000	$ 250,000
Interest rate	5.50%
Period after which debt can be called at par	5 years
Term in Years	45 years	45 years
5.50% issuance 2019 | Maximum
Debt Instrument [Line Items]
Consecutive quarterly periods for which interest payment can be deferred | period	40
7.125% issuance 2023 and 2024
Debt Instrument [Line Items]
Interest rate	7.125%
Period after which debt can be called at par	5 years
Debt instrument, number of loans outstanding | loan	2
Interest rate reset period	5 years
Debt instrument, basis spread on variable rate	3.533%
7.125% issuance 2023 and 2024 | Maximum
Debt Instrument [Line Items]
Consecutive semiannual periods for which interest payment can be deferred | period	20
Fixed-To-Fixed Reset Rate Subordinated Notes Due 2056
Debt Instrument [Line Items]
Total aggregate principal amount	$ 600,000
Interest rate	5.75%
Fixed-To-Fixed Reset Rate Subordinated Notes Due 2056 | Noncallable Period One
Debt Instrument [Line Items]
Total aggregate principal amount	$ 150,000
Subordinated borrowing, noncallable period	5 years
5.95% issuance 2026
Debt Instrument [Line Items]
Total aggregate principal amount	$ 450,000	$ 0
Interest rate	5.95%
Debt instrument, basis spread on variable rate	1.80%
Term in Years	30 years	30 years
5.95% issuance 2026 | Noncallable Period One
Debt Instrument [Line Items]
Subordinated borrowing, noncallable period	10 years
5.75% issuance 2026
Debt Instrument [Line Items]
Total aggregate principal amount		$ 0
Interest rate	5.75%
Debt instrument, basis spread on variable rate	2.00%
Term in Years	30 years	30 years
Subordinated notes
Debt Instrument [Line Items]
Total aggregate principal amount	$ 72,956	$ 43,811
Period after which debt can be called at par	5 years
Consecutive semiannual periods for which interest payment can be deferred | period	20
Term in Years	30 years	30 years