Defined benefit plan percentage changes in discount rate.

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Defined contribution plan age limit of employees to be eligible for catch up contributions.

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- Definition

Defined contribution plan contributions by employer.

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- Definition

Defined contribution plan eligibility minimum service period.

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- Definition

Defined contribution plan employer matching percent of employee contribution.

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- Definition

Percentage of non matching contribution to participants.

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- Definition

Face (par) amount of the original debt instrument that was repurchased.

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- Definition

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

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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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Amount of cost (credit) included in accumulated other comprehensive (income) loss expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

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Amount of contributions expected to be received by defined benefit plan from employer in next fiscal year following latest fiscal year.

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- Definition

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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- Definition

Description of defined benefit pension and other postretirement plans. Includes, but is not limited to, employee group covered, type of benefit formula, type of benefits, cost sharing arrangement, funding policy, type of assets held, significant nonbenefit liabilities and information pertaining to multiple employer plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Percentage employer matches of the employee's percentage contribution matched.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution 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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Retirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Average annual compensation period	Ten years
Defined pension benefit plan, net periodic benefit cost	$ 910,000
Employer contributions	600,000	$ 555,000
Anticipated contribution in the next fiscal year related to pension plans	$ 1,000,000
Increase decrease in the discount rate	1.00%	1.00%
Decrease increase pension expense	$ 170,000	$ 152,000
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	$ 1,159,000	$ 1,118,000	$ 1,057,000
Maximum [Member]
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 [Member]
Defined Benefit Plan Disclosure [Line Items]
Matching contributions by company	50.00%
Contributions by employer per employee's compensation	5.00%
Equity Securities [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset percentage	75.00%	75.00%
Fixed Income Securities [Member] \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, Plan asset percentage	25.00%	25.00%
Yield Curve Technique [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase decrease in the discount rate	1.00%	1.00%
Decrease increase pension expense	$ 243,000	$ 240,000
Yield Curve Technique [Member] \| Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Par amount outstanding	$ 250,000,000