Summary of Defined Benefit Plan's Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	May. 31, 2015	May. 31, 2014	May. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	$ 28,160	$ 26,470	$ 24,442
Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	28,160	26,470
Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	1,775	1,099
Money Market Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	1,775	1,099
Bond Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	11,524	11,570
Bond Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	11,524	11,570
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	14,860	13,801
Equity Funds | Quoted Prices In Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit plan assets fair value	$ 14,860	$ 13,801