Retirement Plans and Other Post-Retirement Benefits - Schedule of Defined Benefit Plan Weighted-Average Assumptions Used in Computing Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate - benefit expense	4.43%	4.65%	4.21%
Future compensation growth rate	3.00%	3.50%	4.00%
Expected long-term rate of return on plan assets	7.25%	7.75%	8.00%
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate - benefit expense	4.18%	4.38%	3.89%