Pension and Other Postretirement Benefits - Contributions for Pension and Other Postretirement Benefits (Detail) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	$ 1,124	$ 901
Other Postretirement Benefit Plans [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	566	560
U.K. [Member] | Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	182	139
U.S. [Member] | Defined Benefit Pension Plan [Member]
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Contributions	$ 1,319	$ 1,248