Subordinated Deferrable Debt - Narrative (Details) - USD ($) $ in Thousands			1 Months Ended				12 Months Ended
	Jun. 26, 2023	May 26, 2023	Apr. 30, 2026	Jul. 31, 2023	Apr. 30, 2023	Mar. 31, 2023	May 31, 2023	May 31, 2022
Maximum
Subordinated Deferrable Debt
Consecutive period for which interest payment can be deferred							5 years
Subordinated debt
Subordinated Deferrable Debt
Period after which debt can be called at par							10 years
Debt instrument, period after which debt can be converted							10 years
Debt instrument, face amount							$ 1,300,000	$ 1,000,000
Subordinated debt | Maximum
Subordinated Deferrable Debt
Term of debt							30 years
4.75% subordinated debt
Subordinated Deferrable Debt
Interest rate						4.75%
Term of debt								30 years
Debt instrument, face amount								$ 400,000
5.25% issuance 2016
Subordinated Deferrable Debt
Interest rate							5.25%
Term of debt							30 years	30 years
Debt instrument, face amount							$ 350,000	$ 350,000
Subordinated notes with variable interest rates due 2043
Subordinated Deferrable Debt
Term of debt							30 years
Debt instrument, face amount							$ 400,000
Subordinated notes with variable interest rates due 2043 | LIBOR
Subordinated Deferrable Debt
Debt instrument, basis spread on variable rate					2.91%
Subordinated notes with variable interest rates due 2043 | Subsequent event
Subordinated Deferrable Debt
Redemption of debt, amount	$ 100,000
Subordinated notes with variable interest rates due 2043 | Subsequent event | SOFR
Subordinated Deferrable Debt
Debt instrument, basis spread on variable rate				0.26161%
Subordinated notes with variable interest rates due 2046 | Forecast
Subordinated Deferrable Debt
Debt instrument, additional adjustment on variable rate			3.63%
Subordinated notes with variable interest rates due 2046 | SOFR | Forecast
Subordinated Deferrable Debt
Debt instrument, basis spread on variable rate			0.26161%
5.50% issuance 2019
Subordinated Deferrable Debt
Interest rate							5.50%
Term of debt							45 years	45 years
Period after which debt can be called at par							5 years
Debt instrument, face amount							$ 250,000	$ 250,000
5.50% issuance 2019 | Maximum
Subordinated Deferrable Debt
Term of debt							45 years
Consecutive period for which interest payment can be deferred							40 years
7.125% issuance 2023
Subordinated Deferrable Debt
Interest rate		7.125%					7.125%
Term of debt							30 years	30 years
Period after which debt can be called at par		5 years
Debt instrument, period after which debt can be converted		5 years
Debt instrument, face amount		$ 300,000					$ 300,000	$ 0
7.125% issuance 2023 | Five-year U.S. Treasury rate
Subordinated Deferrable Debt
Debt instrument, basis spread on variable rate		3.533%
7.125% issuance 2023 | Maximum
Subordinated Deferrable Debt
Term of debt		30 years
Consecutive period for which interest payment can be deferred		20 years