Benefit Costs Recognized as Expense Under Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Benefit plan contributions
Benefit plan contributions	$ 1,142	$ 953
Savings Plan
Benefit plan contributions
Benefit plan contributions	$ 1,142	$ 953

Amount of cost (reversal of cost) for pension and other postretirement benefits.

No definition available.

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