Employee Benefits - Postretirement Estimated Future Benefit Payments (Detail) - Other Postretirement Plan - Other Postretirement Plan $ in Millions	Dec. 31, 2017 USD ($)
Schedule of Pension and Other Postretirement Benefits Expected Benefit Payments [Line Items]
2018	$ 2.7
2019	2.7
2020	2.7
2021	2.6
2022	2.5
2023-2027	$ 11.4

Schedule of Pension and Other Postretirement Benefits Expected Benefit Payments [Line Items]

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- Definition

Amount of benefits for defined benefit plan expected to be paid in five fiscal years after fifth fiscal year following latest fiscal year.

+ 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 (f)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of benefits for defined benefit plan expected to be paid in next fiscal year following latest fiscal year.

+ References

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- Definition

Amount of benefits for defined benefit plan expected to be paid in fifth fiscal year following latest fiscal year.

+ References

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Amount of benefits for defined benefit plan expected to be paid in fourth fiscal year following latest fiscal year.

+ References

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Amount of benefits for defined benefit plan expected to be paid in third fiscal year following latest fiscal year.

+ References

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Amount of benefits for defined benefit plan expected to be paid in second fiscal year following latest fiscal year.

+ References

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