Employer Sponsored Benefit Plan - Assumptions Used in Determining the Year-End Benefit Obligation and Annual Net Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit Obligation
Discount rate	3.71%	4.21%
Expected return on plan assets	5.80%	6.30%
Net Cost
Discount rate	4.21%	4.56%	4.16%
Expected return on plan assets	6.30%	7.00%	7.25%