Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 208	$ 125	$ 466	$ 468
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,566	1,665	4,645	4,988
Expected return on plan assets	(2,320)	(2,323)	(6,962)	(6,966)
Amortization of net actuarial loss	987	738	2,860	2,314
Net periodic benefit cost (income)	233	80	543	336
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	138	151	430	441
Expected return on plan assets	(216)	(203)	(673)	(593)
Amortization of net actuarial loss	53	97	166	284
Net periodic benefit cost (income)	$ (25)	$ 45	$ (77)	$ 132