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Pension Plans and Postretirement Benefits - 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]      
Service cost $ 11,300 $ 9,700 $ 10,200
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 10,650 9,123 9,593
Interest cost 37,199 46,307 47,496
Expected return on plan assets (77,298) (69,809) (80,939)
Amortization of prior service cost 3,370 2,903 2,221
Recognized net actuarial loss (gain) 6,829 9,353 36,738
Net periodic benefit cost (income) (19,250) (2,123) 15,109
Other Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 642 545 620
Interest cost 1,612 2,254 2,121
Expected return on plan assets (2,691) (2,514) (2,540)
Amortization of prior service cost (193) (311) (311)
Recognized net actuarial loss (gain) (458) (5,723) (4,094)
Net periodic benefit cost (income) $ (1,088) $ (5,749) $ (4,204)