Income Taxes (Components of Deferred Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2013	May 31, 2012
Deferred tax assets:
Allowance for doubtful accounts	$ 5,322	$ 5,577
Inventory obsolescence	12,220	11,507
Insurance and contingencies	33,984	28,708
Stock-based compensation	17,513	16,017
Foreign tax credit carry-forward	5,397	9,054
Treasury locks	8,020	9,032
Other	20,030	9,421
Deferred tax assets, gross	102,486	89,316
Valuation allowance	(12,789)	(9,054)
Deferred tax assets, net	89,697	80,262
Deferred tax liabilities:
In service inventory	131,334	64,061
Property	123,904	122,675
Intangibles	99,267	88,696
State taxes and other	22,844	11,970
Deferred tax liabilities	377,349	287,402
Net deferred tax liability	$ 287,652	$ 207,140