Subordinated Deferrable Debt - Additional Information (Details)	12 Months Ended
	May 31, 2021	May 31, 2020
Subordinated Deferrable Debt
Interest rate (as a percent)	5.11%	5.11%
Period after which debt can be called at par	10 years
Debt Instrument, Period after which Debt can be Converted	10 years
Maximum
Subordinated Deferrable Debt
Consecutive period for which interest payment can be deferred	5 years
4.75 percent due 2043
Subordinated Deferrable Debt
Interest rate (as a percent)	4.75%
Term of debt	30 years	30 years
5.25 Percent Due 2046
Subordinated Deferrable Debt
Interest rate (as a percent)	5.25%
Term of debt	30 years	30 years
Subordinated Debt | Maximum
Subordinated Deferrable Debt
Term of debt	30 years
5.50 Percent Due 2064
Subordinated Deferrable Debt
Interest rate (as a percent)	5.50%
Term of debt	45 years	45 years
Period after which debt can be called at par	5 years
5.50 Percent Due 2064 | Maximum
Subordinated Deferrable Debt
Term of debt	45 years
Consecutive period for which interest payment can be deferred	40 years