Subordinated Deferrable Debt (Details) - USD ($) $ in Thousands	12 Months Ended
	May. 31, 2015	May. 31, 2014
Subordinated Deferrable Debt
Subordinated deferrable debt	$ 400,000	$ 400,000
4.75 percent due 2043
Subordinated Deferrable Debt
Period after which debt can be called at par	10 years
Interest rate (as a percent)	4.75%	4.75%
Subordinated deferrable debt	$ 400,000	$ 400,000
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