Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1,661	$ 1,729
Expected return on plan assets	(2,321)	(2,254)
Amortization of net actuarial loss	788	882
Net periodic benefit cost (income)	128	357
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	143	193
Expected return on plan assets	(192)	(238)
Amortization of net actuarial loss	92	20
Net periodic benefit cost (income)	$ 43	$ (25)