Subordinated Deferrable Debt Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	May 31, 2016	May 31, 2015
Subordinated Deferrable Debt
Interest rate (as a percent)	4.98333%	4.75%
4.75 percent due 2043
Subordinated Deferrable Debt
Interest rate (as a percent)	4.75%	4.75%
Period after which debt can be called at par	10 years
4.75 percent due 2043 | Maximum
Subordinated Deferrable Debt
Term of debt	30 years
Consecutive period for which interest payment can be deferred	5 years
5.25 Percent Due 2046
Subordinated Deferrable Debt
Interest rate (as a percent)	5.25%
Proceeds from Subordinated Short-term Debt	$ 350