Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 29	$ (44)	$ 59	$ (88)
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,421	1,661	2,842	3,322
Expected return on plan assets	(2,437)	(2,361)	(4,874)	(4,722)
Amortization of net actuarial loss	1,052	652	2,104	1,304
Net periodic benefit cost (income)	36	(48)	72	(96)
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	107	140	216	282
Expected return on plan assets	(133)	(198)	(267)	(399)
Amortization of net actuarial loss	19	62	38	125
Net periodic benefit cost (income)	$ (7)	$ 4	$ (13)	$ 8