Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 129	$ 172	$ 258	$ 343
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,540	1,662	3,079	3,323
Expected return on plan assets	(2,321)	(2,322)	(4,642)	(4,643)
Amortization of net actuarial loss	936	788	1,873	1,576
Net periodic benefit cost (income)	155	128	310	256
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	144	147	292	290
Expected return on plan assets	(226)	(198)	(457)	(390)
Amortization of net actuarial loss	56	95	113	187
Net periodic benefit cost (income)	$ (26)	$ 44	$ (52)	$ 87