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Postretirement Benefit Plans - Schedule of Amounts Recognized in Balance Sheets (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
United States [Member]    
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Non-current asset $ 3,470 $ 1,479
Current liability (288) (301)
Non-current liability (15,217) (17,254)
Net amount recognized (12,035) (16,076)
United Kingdom [Member]    
Pension Plans, Postretirement and Other Employee Benefits [Line Items]    
Non-current asset 2,978 2,507
Net amount recognized $ 2,978 $ 2,507