Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ (44)	$ 129
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1,661	1,539
Expected return on plan assets	(2,361)	(2,321)
Amortization of net actuarial loss	652	937
Net periodic benefit cost	(48)	155
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	142	148
Expected return on plan assets	(201)	(231)
Amortization of net actuarial loss	63	57
Net periodic benefit cost	$ 4	$ (26)