Stock-Based Compensation - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended
Jan. 31, 2015	Jan. 31, 2014	Jan. 31, 2015	Jan. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option for common stock			337,000	632,500
Stock-based compensation expense	$ 386,000us-gaap_ShareBasedCompensation	$ 382,000us-gaap_ShareBasedCompensation	$ 1,179,000us-gaap_ShareBasedCompensation	$ 1,081,000us-gaap_ShareBasedCompensation
Income tax benefit	141,000	125,000	413,000	353,000
Excess tax benefits from stock based compensation			87,000	349,000
Stock option exercised			149,646	549,804
Intrinsic value of options exercised			575,000	2,000,000
Unrecognized compensation cost related to unvested stock option	$ 3,300,000		$ 3,300,000
Weighted average period for unrecognized compensation cost			1 year 7 months 10 days

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (i)

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- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

No definition available.

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount of cash inflow from realized tax benefit related to deductible compensation cost reported on the entity's tax return for equity instruments in excess of the compensation cost for those instruments recognized for financial reporting purposes.

+ References

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- Definition

The aggregate amount of noncash, equity-based employee remuneration. This may include the value of stock or unit options, amortization of restricted stock or units, and adjustment for officers' compensation. As noncash, this element is an add back when calculating net cash generated by operating activities using the indirect method.

+ References

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- Definition

Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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- Definition

Gross number of share options (or share units) granted during the period.

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No definition available.

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- Definition

Number of share options (or share units) exercised during the current period.

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