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Assumptions To Value Stock Options (Detail)
12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Non-Qualified Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 1.88% 2.28% 2.95%
Expected volatility 50.92% 52.23% 52.88%
Risk-free interest rate 1.88% 1.69% 0.91%
Expected life (years) 6 years 6 years 6 years
Market-Based Restricted Common Shares      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Dividend yield 1.60% 2.28%  
Expected volatility 44.00% 53.40%  
Risk-free interest rate 1.70% 1.41%