Retirement Plans and Other Post-Retirement Benefits - Schedule of Net Periodic Costs of Pension and Post Retirement Medical Benefit Plans Included in Continuing Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 48		$ 92
Interest cost	$ 273	300	$ 523	601
Amortization of prior service cost/(credit)	12	11	24	23
Amortization of actuarial loss	199	161	398	321
Total net periodic benefit expense/(income)	484	520	945	1,037
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	7	14	14
Interest cost	32	46	64	92
Amortization of prior service cost/(credit)	(59)	(116)	(117)	(232)
Amortization of actuarial loss	12	(208)	24	(417)
Total net periodic benefit expense/(income)	$ (8)	$ (271)	$ (15)	$ (543)