Subordinated Deferrable Debt - Subordinated deferrable debt outstanding and weighted-average interest rates (Details) - USD ($) $ in Thousands		1 Months Ended	12 Months Ended
	May 26, 2023	Mar. 31, 2023	May 31, 2023	May 31, 2022
Debt Instrument [Line Items]
Subordinated debt			$ 1,283,436	$ 986,518
Debt, weighted average interest rate			6.64%	5.11%
Subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount			$ 1,300,000	$ 1,000,000
Debt issuance costs			(16,564)	(13,482)
Variable issuance 2013
Debt Instrument [Line Items]
Debt instrument, face amount			$ 400,000
Debt, weighted average interest rate			8.21%
Term of debt			30 years
Period after which debt can be called at par			Apr. 30, 2023
4.75% subordinated debt
Debt Instrument [Line Items]
Debt instrument, face amount				$ 400,000
Interest rate		4.75%
Debt, weighted average interest rate				4.75%
Term of debt				30 years
Period after which debt can be called at par				Apr. 30, 2023
5.25% issuance 2016
Debt Instrument [Line Items]
Debt instrument, face amount			$ 350,000	$ 350,000
Interest rate			5.25%
Debt, weighted average interest rate			5.25%	5.25%
Term of debt			30 years	30 years
Period after which debt can be called at par			Apr. 20, 2026	Apr. 20, 2026
5.50% issuance 2019
Debt Instrument [Line Items]
Debt instrument, face amount			$ 250,000	$ 250,000
Interest rate			5.50%
Debt, weighted average interest rate			5.50%	5.50%
Term of debt			45 years	45 years
Period after which debt can be called at par			May 15, 2024	May 15, 2024
7.125% issuance 2023
Debt Instrument [Line Items]
Debt instrument, face amount	$ 300,000		$ 300,000	$ 0
Interest rate	7.125%		7.125%
Debt, weighted average interest rate			7.13%	0.00%
Term of debt			30 years	30 years
Period after which debt can be called at par			Jun. 15, 2028	Jun. 15, 2028