Summary of Significant Accounting Policies - Interest Expense Table (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2014		Aug. 31, 2013
Interest expense on debt:
Debt issuance costs	$ 1,793	[1]	$ 1,865	[1]
Fee expense	2,035	[2]	2,604	[2]
Total interest expense	156,552		167,585
Short-term debt
Interest expense on debt:
Interest expense on debt	1,274	[3]	1,432	[3]
Medium-term notes
Interest expense on debt:
Interest expense on debt	16,719	[3]	21,571	[3]
Collateral trust bonds
Interest expense on debt:
Interest expense on debt	74,767	[3]	76,798	[3]
Subordinated deferrable debt
Interest expense on debt:
Interest expense on debt	4,750	[3]	4,750	[3]
Subordinated certificates
Interest expense on debt:
Interest expense on debt	16,780	[3]	20,626	[3]
Long-term notes payable
Interest expense on debt:
Interest expense on debt	$ 38,434	[3]	$ 37,939	[3]

[1] Primarily consists of underwriter’s fees, legal fees, printing costs and certain accounting fees, which are deferred and recognized in interest expense using the effective interest method. Also includes issuance costs related to dealer commercial paper, which are recognized immediately as incurred.

[2] Reflects various fees related to funding activities, including fees paid to banks participating in our revolving credit agreements. Amounts are recognized as incurred or amortized on a straight-line basis over the life of the agreement.

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