Postretirement Benefit Plans - Schedule of Amounts Recognized in Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Current liability	$ (166)	$ (198)
Non-current liability	(25,469)	(38,756)
Net amount recognized	(25,635)	(38,954)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Non-current asset		1,475
Non-current liability	(1,698)
Net amount recognized	$ (1,698)	$ 1,475