Subordinated Deferrable Debt - Subordinated deferrable debt outstanding and weighted-average interest rates (Details) - USD ($) $ in Thousands	12 Months Ended
	May 31, 2026	May 31, 2025
Debt Instrument [Line Items]
Total subordinated deferrable debt	$ 1,310,282	$ 1,329,485
Weighted Average Interest Rate	6.19%	6.36%
Term in Years		30 years
Subordinated debt
Debt Instrument [Line Items]
Total aggregate principal amount	$ 1,322,956	$ 1,343,811
Unamortized premiums	3,409	3,450
Debt issuance costs	(16,083)	(17,776)
Variable rate issuance 2013
Debt Instrument [Line Items]
Total aggregate principal amount	$ 0	$ 300,000
Weighted Average Interest Rate	0.00%	7.45%
Term in Years	30 years
5.25% issuance 2016
Debt Instrument [Line Items]
Interest rate	5.25%
Total aggregate principal amount	$ 0	$ 350,000
Weighted Average Interest Rate	0.00%	5.25%
Term in Years	30 years	30 years
5.50% issuance 2019
Debt Instrument [Line Items]
Interest rate	5.50%
Total aggregate principal amount	$ 250,000	$ 250,000
Weighted Average Interest Rate	5.50%	5.50%
Term in Years	45 years	45 years
Period after which debt can be called at par	5 years
7.125% issuance 2023
Debt Instrument [Line Items]
Interest rate	7.125%
Total aggregate principal amount	$ 300,000	$ 300,000
Weighted Average Interest Rate	7.13%	7.13%
Term in Years	30 years	30 years
7.125% issuance 2024
Debt Instrument [Line Items]
Interest rate	7.125%
Total aggregate principal amount	$ 100,000	$ 100,000
Weighted Average Interest Rate	7.13%	7.13%
Term in Years	29 years	29 years
5.75% issuance 2026
Debt Instrument [Line Items]
Interest rate	5.75%
Total aggregate principal amount		$ 0
Weighted Average Interest Rate	5.75%	0.00%
Term in Years	30 years	30 years
5.95% issuance 2026
Debt Instrument [Line Items]
Interest rate	5.95%
Total aggregate principal amount	$ 450,000	$ 0
Weighted Average Interest Rate	5.95%	0.00%
Term in Years	30 years	30 years
Subordinated notes
Debt Instrument [Line Items]
Total aggregate principal amount	$ 72,956	$ 43,811
Weighted Average Interest Rate	5.74%	5.75%
Term in Years	30 years	30 years
Period after which debt can be called at par	5 years