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Postretirement Benefit Plans - Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (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]      
Net actuarial (gain) loss $ 336 $ (5,174) $ 9,348
Amortization of net actuarial loss (4,262) (2,490) (3,814)
Total recognized in other comprehensive income (3,926) (7,664) 5,534
Total recognized in net periodic benefit cost and other comprehensive income (3,743) (8,008) 6,258
United Kingdom [Member]      
Pension Plans, Postretirement and Other Employee Benefits [Line Items]      
Net actuarial (gain) loss 272 (1,144) 325
Amortization of net actuarial loss (79) (244) (219)
Total recognized in other comprehensive income 193 (1,388) 106
Total recognized in net periodic benefit cost and other comprehensive income $ 169 $ (1,377) $ 5