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Employee Future Benefits - Summary of Re-measurement Effects Recognized in Other Comprehensive Income (Detail) - CAD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Pension defined benefit plans [member]    
Disclosure of defined benefit plans [Line Items]    
Experience   $ (15)
Demographic assumption changes $ (35)  
Economic assumption changes 250 (214)
Return on plan assets (excluding interest income) (315) 312
Change in effect of asset limit (9)  
Total re-measurement effects (109) 83
Retiree welfare plans [Member]    
Disclosure of defined benefit plans [Line Items]    
Experience 7 9
Demographic assumption changes 1  
Economic assumption changes 56 (41)
Return on plan assets (excluding interest income) (16) 30
Total re-measurement effects $ 48 $ (2)