Pension and Postretirement Benefits Other Than Pension Plans - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 914	$ 977
Expected return on plan assets	(1,036)	(1,034)
Amortization of net actuarial loss	282	292
Total net periodic pension/postretirement benefit (income) expense	160	235
Other Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	26	30
Amortization of net actuarial loss	57	59
Amortization of prior service credit	(328)	(328)
Total net periodic pension/postretirement benefit (income) expense	$ (245)	$ (239)