Postretirement Benefits Other Than Pensions - Change in Benefit Obligation of Postretirement Plans to Accrued Postretirement Liability (Detail) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	$ 3,214	$ 3,559
Interest cost	108	120	$ 127
Actuarial (gain) loss	264	(47)
Benefits paid	(419)	(418)
Benefit obligation at end of year	$ 3,167	$ 3,214	$ 3,559