Employee Benefits - Postretirement Benefits Other Than Pension Expenses (Detail) - Defined Contribution and Other Retirement Plans [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Service cost for benefits earned	$ 221	$ 214	$ 663	$ 644
Interest cost on projected benefit obligation	588	787	1,764	2,360
Net amortization and deferrals	(1,046)	(1,355)	(3,138)	(4,065)
Net periodic benefit (income) cost	$ (237)	$ (354)	$ (711)	$ (1,061)