General Information and Accounting Policies (Details 4) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2013	Aug. 31, 2012
Interest expense on debt:
Debt issuance costs	$ 1,865	$ 1,937
Fee expense	2,604	2,162
Total interest expense	167,585	176,596
Short-term debt
Interest expense on debt:
Interest expense on debt	1,432	1,619
Medium-term notes
Interest expense on debt:
Interest expense on debt	21,571	27,883
Collateral trust bonds
Interest expense on debt:
Interest expense on debt	76,798	81,439
Subordinated deferrable debt
Interest expense on debt:
Interest expense on debt	4,750	2,806
Subordinated certificates
Interest expense on debt:
Interest expense on debt	20,626	20,354
Long-term notes payable
Interest expense on debt:
Interest expense on debt	$ 37,939	$ 38,396