Income Taxes (Components of Deferred Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2014	May 31, 2013	May 31, 2012
Deferred tax assets:
Allowance for doubtful accounts	$ 4,835	$ 5,322
Inventory obsolescence	11,682	12,220
Insurance and contingencies	36,032	33,984
Stock-based compensation	23,890	17,513
Foreign tax credit carry-forward	4,266	5,397
Treasury locks	6,924	8,020
Other	30,526	20,030
Deferred tax assets, gross	118,155	102,486
Valuation allowance	(13,358)	(12,789)	(9,054)
Deferred tax assets, net	104,797	89,697
Deferred tax liabilities:
In service inventory	150,439	131,334
Property	90,155	123,904
Intangibles	81,215	99,267
Investment in partnerships	93,227	4,025
State taxes and other	24,650	18,819
Deferred tax liabilities	439,686	377,349
Net deferred tax liability	$ 334,889	$ 287,652