Pension and Other Postretirement Benefits - Contributions for Pension and Other Postretirement Benefits (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	$ 1,308	$ 1,323
Other Postretirement Benefit Plans [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	880	852
U.K. [Member] | Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	270	223
U.S. [Member] | Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	$ 1,991	$ 1,788