Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	May 31, 2017	May 31, 2016	May 31, 2015
Deferred tax assets:
Allowance for doubtful accounts	$ 7,707	$ 7,416
Inventory obsolescence	16,096	13,702
Insurance and contingencies	54,489	42,717
Stock-based compensation	73,027	45,720
Net operating loss and foreign related carry-forwards	37,814	17,883
Treasury locks	0	12,055
Other	25,891	8,100
Deferred tax assets, gross	215,024	147,593
Valuation allowance	(18,088)	(17,047)	$ (14,690)
Deferred tax assets, net	196,936	130,546
Deferred tax liabilities:
In service inventory	210,766	172,704
Property	126,872	93,784
Intangibles	290,049	104,585
Treasury locks	6,435	0
State taxes and other	32,142	18,948
Deferred tax liabilities	666,264	390,021
Net deferred tax liability	$ 469,328	$ 259,475