Summary of Defined Benefit Plan's Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	May 31, 2017	May 31, 2016	May 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	$ 27,022	$ 25,566	$ 28,159
Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	27,022	25,566
Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	294	340
Money Market Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	294	340
Bond Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	14,613	12,435
Bond Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	14,613	12,435
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	12,115	12,791
Equity Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	$ 12,115	$ 12,791