Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ (236)	$ 29	$ (477)	$ 59
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,177	1,421	2,354	2,842
Expected return on plan assets	(2,586)	(2,437)	(5,171)	(4,874)
Amortization of net actuarial loss	1,144	1,052	2,288	2,104
Net periodic benefit cost (income)	(265)	36	(529)	72
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	91	107	179	216
Expected return on plan assets	(81)	(133)	(162)	(267)
Amortization of net actuarial loss	18	19	35	38
Net periodic benefit cost (income)	$ 28	$ (7)	$ 52	$ (13)