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Health Care Cost Trend Rates (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Pension And Other Post-Retirement Benefits    
Health care cost trend rate assumed for next year 6.50% 6.50%
Rate to which the cost trend rate is assumed to decline (the ultimate trend rate) 5.00% 5.00%
Year that the rate reaches the ultimate trend rate 2021 2021