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Pensions and Other Benefits - Summary of Impact of One-Percentage Point Change in Assumed Health Care Cost Trend Rates (Detail)
12 Months Ended
Dec. 31, 2017
CAD
Retirement Benefits [Abstract]  
Effect of one percentage point increase on the total of service and interest costs CAD 0
Effect of one percentage point decrease on the total of service and interest costs 0
Effect of one percentage point increase on post-retirement benefit obligation (6,000,000)
Effect of one percentage point decrease on post-retirement benefit obligation CAD 6,000,000