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Postretirement Benefit Plans - Amounts Recognized in Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
United States [Member]    
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Net actuarial loss $ 33,745 $ 37,671
United Kingdom [Member]    
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Net actuarial loss $ 4,653 $ 4,460