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Pension and Other Benefit Plans - Defined Benefit Plan Weighted Average Assumptions Used in Calculating Benefit Obligation and Net Periodic Benefit Cost For US Postretirement Medical Plan (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 2.60% 3.30%  
Current medical cost trend rate 7.25% 7.50%  
Ultimate medical cost trend rate 4.50% 4.50%  
Year that the rate reaches the ultimate trend rate 2031 2031  
Postretirement medical plan      
Defined Benefit Plan Disclosure [Line Items]      
Discount rate 3.30% 4.30% 3.70%
Return on plan assets 7.00% 7.00% 7.00%
Current medical cost trend rate 7.25% 7.50% 7.00%
Ultimate medical cost trend rate 4.50% 4.50% 5.00%
Year that the rate reaches the ultimate trend rate 2031 2031 2026