Pension and Postretirement Benefit Plans - Additional Information (Detail) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Retirement Benefits [Abstract]
Contributions by employer	$ 9.8

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

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration