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Postretirement Benefit Plans - Company's Asset Allocations for its U.K. Pension Plans by Asset Category (Detail) - United Kingdom [Member] - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Total $ 28,504 $ 24,357 $ 20,576
Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total   252  
Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 27,345 22,900  
Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 1,159 1,205  
Cash [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 2,576 1,372  
Cash [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total   252  
Cash [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 2,576 1,120  
Pooled Pension Funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 3,603 7,533  
Pooled Pension Funds [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 3,603 7,533  
Pooled Fixed Pension Funds [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 21,166 14,247  
Pooled Fixed Pension Funds [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 21,166 14,247  
Insurance Contracts [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total 1,159 1,205  
Insurance Contracts [Member] | Level 3 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Total $ 1,159 $ 1,205