Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Net periodic (benefit) cost	$ 30	$ (44)
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,421	1,661
Expected return on plan assets	(2,437)	(2,361)
Amortization of net actuarial loss	1,052	652
Net periodic (benefit) cost	36	(48)
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	109	142
Expected return on plan assets	(134)	(201)
Amortization of net actuarial loss	19	63
Net periodic (benefit) cost	$ (6)	$ 4