Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 6,651	$ 6,934	$ 6,815
Expected return on plan assets	(9,288)	(9,012)	(9,579)
Amortization of net actuarial loss	3,085	3,386	4,534
Net periodic benefit cost	448	1,308	1,770
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	592	733	789
Expected return on plan assets	(797)	(900)	(1,054)
Amortization of net actuarial loss	382	77	179
Net periodic benefit cost	$ 177	$ (90)	$ (86)