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Note 4 - Stock-based Compensation (Details Textual) (USD $)
9 Months Ended
Jan. 31, 2015
Share-based Compensation [Abstract]  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross 276,800us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price $ 17.84us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period 36,700us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price $ 3.92us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period 7,800us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod
Share-based Compensation Arrangement by Share-based Payment Award, Options, Forfeitures and Expirations in Period, Weighted Average Exercise Price $ 17.35us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number 636,655us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price $ 11.17us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 2,769,089us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant