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Share-Based Payments (Details) (USD $)
12 Months Ended
Apr. 30, 2011
Summary of the Company's stock option activity and related information 
Outstanding at May 1, 2010711,987
Weighted Average-Exercise Price Outstanding at May 1, 2010$ 41.06
Options Exercised(515,062)
Weighted Average-Exercise Price Exercised$ 41.01
Outstanding and exercisable at April 30, 2011196,925
Weighted Average-Exercise Price Outstanding and exercisable at April 30, 2011$ 41.18