Retirement Plans (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Defined benefit plans, net periodic benefit cost [Abstract]
Service cost	$ 242	$ 251	$ 472	$ 503
Interest cost	6,346	6,506	12,598	13,704
Expected return on plan assets	(9,782)	(8,656)	(19,439)	(18,031)
Net amortization of prior service cost	(23)	(24)	(48)	(49)
Recognized net actuarial loss	1,534	1,270	3,035	2,632
Pension plan actuarial loss	0	8,842	0	8,842
Net pension expense (credit)	(1,683)	8,189	(3,382)	7,601
Employer defined benefit pension plan contributions	2,800	3,000	5,600	10,600
Contributions for employer defined contribution plans	$ 3,000	$ 3,500	$ 7,900	$ 8,400

Amount of gain (loss), recognized immediately as component of net periodic benefit (cost) credit, for change in value of benefit obligation or plan assets from experience different from that assumed, change in actuarial assumption, or consequence of temporarily deviating from substantive plan. Includes, but is not limited to, amount in excess of ten percent of greater of fair value of plan assets or benefit obligation.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 30
-Section 35
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e11661-114930

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- Definition

Amount of gain (loss) recognized in net periodic benefit (cost) credit of defined benefit plan.

+ References

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- Definition

Amount of prior service cost (credit) recognized in net periodic benefit cost (credit) of defined benefit plan.

+ References

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(ii)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2709-114920

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

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(5)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of expected return (loss) recognized in net periodic benefit (cost) credit, calculated based on expected long-term rate of return and market-related value of plan assets of defined benefit plan.

+ References

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- Definition

Amount of cost recognized for passage of time related to defined benefit plan.

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- Definition

Amount of net periodic benefit cost (credit) for defined benefit plan.

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- Definition

Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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- Definition

Amount of discretionary contributions made by an employer to a defined contribution plan.

+ References

No definition available.

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