Changes in Each Component of Accumulated Other Comprehensive Income (Loss) (Parenthetical) (Detail) - USD ($) $ in Thousands	12 Months Ended
Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Tax effects on unrealized (losses) gains	$ (2,300)	$ 70	$ (3,800)
Tax effects on reclassifications of realized net losses	$ 1,200	$ 1,000	$ 900

Amount of tax (expense) benefit, before reclassification adjustments, related to (increase) decrease in accumulated other comprehensive (income) loss related to pension and other postretirement benefit plans.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 220

-SubTopic 10

-Section 45

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e640-108580

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration