Retirement Plans - Assumptions Used in Accounting for Plans (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to calculate benefit obligation	3.40%	3.80%	4.00%
Discount rate used to calculate net periodic pension cost	3.84%	4.00%	3.65%
Executive Compensation [Member]
Defined Benefit Plan Disclosure [Line Items]
Average annual increase in compensation	3.25%	2.50%	2.50%
Director Compensation [Member]
Defined Benefit Plan Disclosure [Line Items]
Average annual increase in compensation	0.00%	2.50%	2.50%