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Pension Plans and Postretirement Benefits - Assumed Health Care Cost Trend Rates (Detail)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Retirement Benefits [Abstract]    
Health care cost trend rate assumed for next year 6.75% 7.00%
Rates to which the cost trend rate is assumed to decline (ultimate trend rate) 4.50% 4.50%
Year that the rate reaches ultimate trend rate 2030 2030