Weighted Average Assumptions Used in Determining Net Periodic Benefit Cost for Pension (Detail) - Pension Benefits	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.90%	4.10%	4.00%
Expected rate of return on plan assets	7.50%	7.50%	8.00%