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Stockholders' Equity (Details 1) (USD $)
 6 Months Ended
Oct. 31, 2012
Oct. 31, 2011
Number of shares and weighted average exercise prices    
Options outstanding, beginning of year, Shares 3,988,164 3,137,565
Options outstanding, beginning of year, Weighted-Average Exercise Price $ 4.67 $ 4.73
Granted during year, Shares 3,500 1,094,100
Granted during year, Weighted-Average Exercise Price $ 11.02 $ 3.12
Exercised during year, Shares (834,508) (123,500)
Exercised during year, Weighted-Average Exercise Price $ 4.10 $ 1.64
Canceled/forfeited during year, Shares (105,496) (205,333)
Canceled/forfeited during year, Weighted-Average Exercise Price $ 3.91 $ 4.89
Options outstanding, end of year, Shares 3,051,660 3,902,832
Options outstanding, end of year, Weighted-Average Exercise Price $ 4.86 $ 4.37
Weighted average remaining contractual life 6 years 7 months 21 days 6 years 1 month 13 days
Options exercisable, end of period, Shares 1,890,540 2,084,359
Options exercisable, end of period, Weighted-Average Exercise Price $ 5.10 $ 5.08
Weighted average remaining contractual life 5 years 4 months 2 days 5 years 2 months 12 days