Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 60	$ (159)	$ 119	$ (247)
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,409	1,640	4,251	4,962
Expected return on plan assets	(2,435)	(2,365)	(7,309)	(7,087)
Amortization of net actuarial loss	1,092	563	3,196	1,867
Net periodic benefit cost (income)	66	(162)	138	(258)
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	111	134	327	416
Expected return on plan assets	(137)	(190)	(404)	(589)
Amortization of net actuarial loss	20	59	58	184
Net periodic benefit cost (income)	$ (6)	$ 3	$ (19)	$ 11