Postretirement Benefit Plans - Obligations and Funded Status (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]
Benefit obligation at beginning of year	$ 162,727	$ 160,789
Interest cost	6,651	6,934	$ 6,815
Actuarial (gain) loss	9,109	1,538
Benefits paid	(7,129)	(6,534)
Benefit obligation at end of year	171,358	162,727	160,789
Fair value of plan assets at beginning of year	137,092	121,835
Actual return on plan assets	15,533	21,612
Employer contributions	2,412	179
Benefits paid	(7,129)	(6,534)
Fair value of plan assets at end of year	147,908	137,092	121,835
Over (Under) funded status at end of year	(23,450)	(25,635)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	22,034	19,950
Interest cost	592	733	789
Actuarial (gain) loss	(156)	5,614
Benefits paid	(516)	(513)
Foreign exchange impact	2,094	(3,750)
Benefit obligation at end of year	24,048	22,034	19,950
Fair value of plan assets at beginning of year	20,336	21,425
Actual return on plan assets	1,957	2,758
Employer contributions	365	378
Benefits paid	(516)	(513)
Foreign exchange impact	2,026	(3,712)
Fair value of plan assets at end of year	24,168	20,336	$ 21,425
Over (Under) funded status at end of year	$ 120	$ (1,698)