Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ (241)	$ 30
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,177	1,421
Expected return on plan assets	(2,586)	(2,437)
Amortization of net actuarial loss	1,144	1,052
Net periodic benefit cost	(265)	36
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	88	109
Expected return on plan assets	(81)	(134)
Amortization of net actuarial loss	17	19
Net periodic benefit cost	$ 24	$ (6)