Retirement Plans and Other Postretirement Benefits - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Employee hire date no longer eligible for noncontributory retirement plan	Dec. 08, 2009
Defined Benefit Plan, Expected Long-Term Rate of Return	7.50%
Equity [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	60.00%
Debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	40.00%
Supplemental Employee Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Deferred Compensation Arrangement with Individual, Compensation Expense	$ 222	$ 52	$ 285
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed discount rate	4.00%	4.30%
Settlements	$ 0	$ (3,166)
Settlement cost	0	$ 1,434	0
Expected employer contributions for next fiscal year	$ 158
Defined Benefit Plan, Expected Long-Term Rate of Return	7.50%	7.50%
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Assumed discount rate	4.00%	4.30%
Settlements	$ 0	$ 0
Settlement cost	0	$ 0	0
Expected employer contributions for next fiscal year	$ 80
Defined Benefit Plan, Expected Long-Term Rate of Return	0.00%	0.00%
Deferred Salary Savings Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Contribution Plan, Cost	$ 1,400	$ 1,200	$ 1,000