Stock Options and Share-Based Compensation - Summary of Options Outstanding, Exercisable, and Weighted Average Exercise Prices within Exercise Price Range (Detail) (USD $)					12 Months Ended
Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011	Apr. 30, 2010	Apr. 30, 2013 Exercise Price Range Range One [Member]	Apr. 30, 2013 Exercise Price Range Range Two [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price range lower limit					$ 8.59	$ 14.69
Exercise price range upper limit					$ 12.66	$ 14.90
Options outstanding	295,550	298,050	279,800	158,925	222,700	72,850
Weighted average exercise price	$ 11.84	$ 11.60	$ 11.94	$ 12.86	$ 10.87	$ 14.79
Weighted average remaining contractual life					6 years 6 months 29 days	4 years 10 months 2 days
Aggregate intrinsic value					$ 485,000
Options exercisable					84,400	72,850
Weighted average exercise price					$ 11.28	$ 14.79
Aggregate intrinsic value					$ 150,000

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of options outstanding.

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The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

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The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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