Retirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
Apr. 30, 2019	Apr. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined pension benefit plan, net periodic benefit cost	$ 970,000
Employer contributions	1,000,000	$ 600,000
Anticipated contribution in the next fiscal year related to pension plans	$ 0
Increase decrease in the discount rate	1.00%	1.00%
Decrease increase pension expense	$ 187,000	$ 170,000
Age limit of employees to be eligible for contribution	21 years
Minimum service period of employees to be eligible for contribution	3 months
Employee's compensation	3.00%
Non-matching contribution for participants employed	1.00%
Contribution made by company	$ 1,291,000	$ 1,159,000
Maximum
Defined Benefit Plan Disclosure [Line Items]
Matching contributions by company	100.00%
Contributions by employer per employee's compensation	3.00%
Employee's compensation	4.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Matching contributions by company	50.00%
Contributions by employer per employee's compensation	5.00%
Equity Securities \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset percentage	75.00%	75.00%
Fixed Income \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset percentage	25.00%	25.00%
Yield Curve Technique
Defined Benefit Plan Disclosure [Line Items]
Increase decrease in the discount rate	1.00%	1.00%
Decrease increase pension expense	$ 234,000	$ 243,000
Yield Curve Technique \| Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Par amount outstanding	$ 250,000,000

Defined benefit plan percentage changes in discount rate.

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Amount of contribution received by defined benefit plan from employer which increases plan assets.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Amount of increase (decrease) in obligation for pension and other postretirement benefits. Includes, but is not limited to, defined benefit and defined contribution plans.

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