Common Stock and Stock-Based Compensation - Weighted Average Assumptions Used to Estimate Fair Values (Detail) - Stock Options	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	1.20%	1.30%	1.50%
Expected volatility	23.60%	27.90%	35.00%
Weighted average expected life (in years)	5 years 9 months	5 years 3 months	5 years 3 months
Risk-free interest rate	1.70%	1.30%	0.60%