Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Finite-Lived Intangible Assets [Line Items]
Amortization expense	$ 3.8	$ 3.7	$ 3.4
Amortization expense, next 12 months	3.0
Amortization expense, 2017	3.0
Amortization expense, 2018	3.0
Amortization expense, 2019	3.0
Amortization expense, 2020	$ 3.0