Summary of Significant Accounting Policies - Interest Expense Table (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Feb. 29, 2016	Feb. 28, 2015	Feb. 29, 2016	Feb. 28, 2015
Interest expense on debt:
Total interest expense		$ 171,189	$ 156,850	$ 504,013	$ 471,677
Short-term debt
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	4,387	2,982	10,311	11,786
Medium-term notes
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	21,773	17,774	62,745	52,640
Collateral trust bonds
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	83,810	79,026	248,410	232,290
Subordinated deferrable debt
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	4,785	4,782	14,356	14,352
Subordinated certificates
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	15,022	15,280	45,425	48,131
Long-term notes payable
Interest expense on debt:
Interest expense on debt	[1],[2],[3]	$ 41,412	$ 37,006	$ 122,766	$ 112,478

[1] Includes 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.

[2] Includes 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.

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