Postretirement Benefit Plans - Obligations and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
United States [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	$ 176,499	$ 158,594
Interest cost	5,668	6,616	$ 6,194
Actuarial loss	13,327	19,251
Benefits paid	(8,123)	(7,962)
Benefit obligation at end of year	187,371	176,499	158,594
Fair value of plan assets at beginning of year	160,423	134,198
Actual return on plan assets	22,738	33,875
Employer contributions	298	312
Benefits paid	(8,123)	(7,962)
Fair value of plan assets at end of year	175,336	160,423	134,198
Over (Under) funded status at end of year	(12,035)	(16,076)
United Kingdom [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	21,850	20,006
Interest cost	445	554	565
Actuarial loss	3,032	1,045
Benefits paid	(657)	(592)
Foreign exchange impact	856	837
Benefit obligation at end of year	25,526	21,850	20,006
Fair value of plan assets at beginning of year	24,357	20,576
Actual return on plan assets	3,320	2,963
Employer contributions	526	476
Benefits paid	(657)	(592)
Foreign exchange impact	958	934
Fair value of plan assets at end of year	$ 28,504	24,357	20,576
Over (Under) funded status at end of year		$ 2,507	$ 2,978