Summary of Significant Accounting Policies (Details 1) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Feb. 28, 2014		Feb. 28, 2013		Feb. 28, 2014		Feb. 28, 2013
Interest expense on debt:
Debt issuance costs	$ 1,806	[1]	$ 1,891	[1]	$ 5,453	[1]	$ 5,733	[1]
Fee expense	2,206	[2]	2,027	[2]	6,925	[2]	6,534	[2]
Total interest expense	163,534		171,899		496,464		522,796
Short-term debt
Interest expense on debt:
Interest expense on debt	1,406	[3]	1,717	[3]	4,445	[3]	5,033	[3]
Medium-term notes
Interest expense on debt:
Interest expense on debt	20,369	[3]	21,294	[3]	62,920	[3]	74,010	[3]
Collateral trust bonds
Interest expense on debt:
Interest expense on debt	76,090	[3]	84,197	[3]	227,746	[3]	247,907	[3]
Subordinated deferrable debt
Interest expense on debt:
Interest expense on debt	4,750	[3]	2,806	[3]	14,250	[3]	8,419	[3]
Subordinated certificates
Interest expense on debt:
Interest expense on debt	19,777	[3]	20,345	[3]	60,897	[3]	61,227	[3]
Long-term notes payable
Interest expense on debt:
Interest expense on debt	$ 37,130	[3]	$ 37,622	[3]	$ 113,828	[3]	$ 113,933	[3]

[1] Includes amortization of all deferred charges related to the issuance of debt, principally underwriters’ fees, legal fees, printing costs and comfort letter fees. Amortization is calculated using the effective interest method or a method approximating the effective interest method. Also includes issuance costs related to dealer commercial paper, which are recognized as incurred.

[2] Includes various fees related to funding activities, including fees paid to banks participating in our revolving credit agreements.

[3] Represents interest expense and the amortization of discounts on debt.