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Pensions and Other Benefits - Actuarial Assumptions Used (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Postretirement Benefits [Line Items]      
Discount rate 3.80% 4.02% 4.22%
Projected future salary increases 2.75% 2.75% 3.00%
Health care cost trend rate 7.00% 7.00% 7.00%
Discount rate 4.02% 4.22% 4.09%
Expected rate of return on fund assets 7.75% 7.75% 7.75%
Projected future salary increases 2.75% 3.00% 3.00%
Health care cost trend rate change 0.50% 0.50%  
Ultimate rate 5.00% 5.00%  
Health care cost trend rate previously assumed   6.50%  
Heath care cost trend rate previously assumed two years prior     7.00%