Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Defined Contribution and Other Retirement Plans [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 196	$ 221	$ 587	$ 663
Interest cost on projected benefit obligation	654	588	1,962	1,764
Net amortization and deferrals	(1,189)	(1,046)	(3,567)	(3,138)
Net periodic benefit income	$ (339)	$ (237)	$ (1,018)	$ (711)