Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 524	$ 661	$ 756
Interest cost	1,927	2,071	1,953
Expected return on plan assets	(3,074)	(3,041)	(3,100)
Amortization of net actuarial loss	1,185	1,296	1,308
Accretion of prior service cost	(282)	(283)	(280)
Settlement cost	0	1,434	0
Net periodic benefit (income) cost	280	2,138	637
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	48	46	59
Interest cost	118	133	110
Expected return on plan assets	0	0	0
Amortization of net actuarial loss	42	25	54
Accretion of prior service cost	0	0	0
Settlement cost	0	0	0
Net periodic benefit (income) cost	$ 208	$ 204	$ 223