Pension Information - Additional Information (Detail) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Additional benefits under pension plan	$ 0
Expected future employer contributions	$ 0
Employer contributions		$ 2,000,000

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