Retirement Plans - Schedule of Actuarial Assumptions Used in Determination of Net Periodic Pension Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Weighted average discount rate	3.96%	4.19%	3.85%
Rates of increase in compensation levels	3.50%	3.50%	3.50%
Expected long-term rate of return on assets	7.25%	7.25%	7.25%