Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	May. 31, 2015	May. 31, 2014	May. 31, 2013
Deferred tax assets:
Allowance for doubtful accounts	$ 4,857	$ 4,835
Inventory obsolescence	12,266	11,682
Insurance and contingencies	38,522	36,032
Stock-based compensation	29,910	23,890
Foreign related carry-forwards	16,862	19,129
Treasury locks	5,829	6,924
Other	9,461	15,663
Deferred tax assets, gross	117,707	118,155
Valuation allowance	(14,690)	(13,358)	$ (12,789)
Deferred tax assets, net	103,017	104,797
Deferred tax liabilities:
In service inventory	169,629	150,439
Property	77,871	90,155
Intangibles	84,218	81,215
Investment in partnerships	86,098	93,227
State taxes and other	24,528	24,650
Deferred tax liabilities	442,344	439,686
Net deferred tax liability	$ 339,327	$ 334,889