Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ 129	$ 171
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,539	1,661
Expected return on plan assets	(2,321)	(2,321)
Amortization of net actuarial loss	937	788
Net periodic benefit cost (income)	155	128
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	148	143
Expected return on plan assets	(231)	(192)
Amortization of net actuarial loss	57	92
Net periodic benefit cost (income)	$ (26)	$ 43