Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,662	$ 1,730	$ 3,323	$ 3,459
Expected return on plan assets	(2,322)	(2,254)	(4,643)	(4,508)
Amortization of net actuarial loss	788	882	1,576	1,764
Net periodic benefit cost (income)	128	358	256	715
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	147	194	290	387
Expected return on plan assets	(198)	(238)	(390)	(476)
Amortization of net actuarial loss	95	20	187	40
Net periodic benefit cost (income)	$ 44	$ (24)	$ 87	$ (49)