Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial (gain) loss	$ 9,348	$ 2,864	$ (11,062)
Amortization of net actuarial loss	(3,814)	(3,085)	(3,386)
Total recognized in other comprehensive income	5,534	(221)	(14,448)
Total recognized in net periodic benefit cost and other comprehensive income	6,258	227	(13,140)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial (gain) loss	325	(1,318)	3,756
Amortization of net actuarial loss	(219)	(382)	(77)
Total recognized in other comprehensive income	106	(1,700)	3,679
Total recognized in net periodic benefit cost and other comprehensive income	$ 5	$ (1,523)	$ 3,589