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Stockholders' Equity (Details 2)
12 Months Ended
May 31, 2014
May 31, 2013
May 31, 2012
Schedule of weighted-average fair value assumptions      
Expected stock price volatility 34.40% 43.91% 49.06%
Risk-free interest rate 1.44% 0.62% 0.70%
Expected life of options 4 years 8 months 25 days 4 years 7 months 13 days 4 years 7 months 2 days