Postretirement Benefits Other Than Pension - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 24	$ 26
Amortization of net actuarial loss	64	57
Amortization of prior service credit	(328)	(328)
Total net periodic pension (income) expense	$ (240)	$ (245)