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Postretirement Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost $ 874 $ 351 $ 1,215
United States [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Interest cost 5,668 6,616 6,194
Expected return on plan assets (9,747) (9,450) (9,284)
Amortization of net actuarial loss 4,262 2,490 3,814
Net periodic benefit cost 183 (344) 724
United Kingdom [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Interest cost 445 554 565
Expected return on plan assets (548) (787) (885)
Amortization of net actuarial loss 79 244 219
Net periodic benefit cost $ (24) $ 11 $ (101)