Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Income) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 477	$ 436	$ 560
Interest cost	1,692	1,905	1,760
Expected loss on plan assets	(3,291)	(3,061)	(3,287)
Amortization of net actuarial loss	1,176	1,176	1,120
Accretion of prior service cost	0	(181)	(283)
Net periodic benefit cost (income)	54	275	(130)
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	109	67	88
Interest cost	96	94	92
Expected loss on plan assets	0	0	0
Amortization of net actuarial loss	26	0	4
Accretion of prior service cost	0	0	0
Net periodic benefit cost (income)	$ 231	$ 161	$ 184