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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 61	$ 216	$ 153	$ 1,073
Interest cost	318	3,287	919	10,074
Expected return on plan assets		(3,895)		(11,290)
Amortization of prior service cost/(credit)	12	64	35	152
Amortization of unrecognized/actuarial loss (gain)	164	819	485	2,383
Total net periodic benefit cost/(credit)	555	491	1,592	2,392
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	8	7	22	25
Interest cost	46	67	138	235
Amortization of prior service cost/(credit)	(115)	(2)	(347)	(7)
Amortization of unrecognized/actuarial loss (gain)	(209)	(293)	(626)	(639)
Total net periodic benefit cost/(credit)	$ (270)	$ (221)	$ (813)	$ (386)