Stock Options and Share-Based Compensation - Fair Value of Options Granted Using Black-Scholes Model (Detail) - $ / shares	12 Months Ended
Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted	45,200	40,200	45,800
2008 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted	45,200	40,200	35,800
Weighted average expected stock price volatility	29.17%	30.45%	46.67%
Expected option life	6 years 3 months	6 years 3 months	6 years 3 months
Average risk-free interest rate	2.10%	1.97%	1.91%
Average dividend yield	3.00%	2.78%	2.61%
Estimated fair value of each option	$ 5.04	$ 3.97	$ 6.60
2010 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted			10,000
Weighted average expected stock price volatility			30.37%
Expected option life			3 years 4 months 17 days
Average risk-free interest rate			1.28%
Average dividend yield			2.69%
Estimated fair value of each option			$ 3.04

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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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

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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