Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 1,215	$ 1,035	$ 1,637
United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	6,194	6,651	6,934
Expected return on plan assets	(9,284)	(9,288)	(9,012)
Amortization of net actuarial loss	3,814	3,085	3,386
Net periodic benefit cost	724	448	1,308
United Kingdom [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	565	592	733
Expected return on plan assets	(885)	(797)	(900)
Amortization of net actuarial loss	219	382	77
Net periodic benefit cost	$ (101)	$ 177	$ (90)