Postretirement Benefit Plans - Schedule of Amounts Recognized in Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current liability	$ (302)	$ (166)
Non-current liability	(23,148)	(25,469)
Net amount recognized	(23,450)	(25,635)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Non-current asset	120
Non-current liability		(1,698)
Net amount recognized	$ 120	$ (1,698)