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, 2017	Dec. 31, 2016	Dec. 31, 2015
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial (gain) loss	$ 2,864	$ (11,062)	$ (1,110)
Amortization of net actuarial loss	(3,085)	(3,386)	(4,534)
Total recognized in other comprehensive income	(221)	(14,448)	(5,644)
Total recognized in net periodic benefit cost and other comprehensive income	227	(13,140)	(3,874)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net actuarial (gain) loss	(1,318)	3,756	(781)
Amortization of net actuarial loss	(382)	(77)	(179)
Total recognized in other comprehensive income	(1,700)	3,679	(960)
Total recognized in net periodic benefit cost and other comprehensive income	$ (1,523)	$ 3,589	$ (1,046)