Deferred Compensation and Retirement Plans - Components of Net Periodic Benefits Costs (Parenthetical) (Detail)	6 Months Ended
Oct. 31, 2017	Oct. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return on plan assets	6.50%	6.50%

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration