Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	May 31, 2016	May 31, 2015	May 31, 2014
Deferred tax assets:
Allowance for doubtful accounts	$ 7,416	$ 4,857
Inventory obsolescence	13,702	12,266
Insurance and contingencies	42,717	38,522
Stock-based compensation	45,720	29,910
Foreign related carry-forwards	17,883	16,862
Treasury locks	12,055	5,829
Other	8,100	9,461
Deferred tax assets, gross	147,593	117,707
Valuation allowance	(17,047)	(14,690)	$ (13,358)
Deferred tax assets, net	130,546	103,017
Deferred tax liabilities:
In service inventory	172,704	169,629
Property	93,784	77,871
Intangibles	104,585	84,218
Investment in partnerships	2,563	86,098
State taxes and other	16,385	24,528
Deferred tax liabilities	390,021	442,344
Net deferred tax liability	$ 259,475	$ 339,327