Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,665	$ 1,741	$ 4,988	$ 5,200
Expected return on plan assets	(2,323)	(2,251)	(6,966)	(6,759)
Amortization of net actuarial loss	738	776	2,314	2,540
Net periodic benefit (income) cost	80	266	336	981
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	151	177	441	564
Expected return on plan assets	(203)	(218)	(593)	(694)
Amortization of net actuarial loss	97	18	284	58
Net periodic benefit (income) cost	$ 45	$ (23)	$ 132	$ (72)