Employee Benefit Plans (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) participant	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Cost recognized	$ 1,097	$ 1,032	$ 1,029
Requisite service period	5 years
Effect of one percentage point increase in assumed health care costs, affect on accumulated benefit obligation	$ 124
Effect of one percentage point decrease in assumed health care costs, effect on accumulated benefit obligation	(116)
Effect of one percentage point increase in assumed health care costs, affect on service and interest cost	6
Effect of one percentage point decrease in assumed health care costs, affect on service and interest cost	$ (6)
Effect On Accumulated Benefit Obligation And Service And Interest Cost [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Ultimate trend rate (as a percent)	1.00%
Post-Employment Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Number of participants | participant	196
Number of active participants eligible for benefits | participant	27
Minimum age of participant to become eligible for benefits	62 years
Age up to which health benefits continuation paid	65 years
Expected contributions in next fiscal year, net of Medicare Part D subsidy receipts	$ 520
Post-Employment Benefit Plan [Member] | Medicare Part D Subsidy Receipts [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Medicare Part D subsidy receipts to the plan in next fiscal year	$ 18