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Common shares - Common Share Activities (Detail) - CAD ($)
shares in Thousands, $ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Common stock beginning balance, (in shares)     509,045    
Common stock, ending balance, (in shares) 496,861   496,861   509,045
Common stock beginning balance [1]     $ 942    
Common stock, ending balance [1] $ 919   $ 919   $ 942
Common Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Common stock beginning balance, (in shares)     509,045 535,837 535,837
Purchases at stated value, (in shares)     (12,184)   (26,792)
Common stock, ending balance, (in shares) 496,861   496,861   509,045
Common stock beginning balance     $ 942 $ 992 $ 992
Purchases at stated value $ (23) $ (23) (23) $ (23) (50)
Common stock, ending balance $ 919   $ 919   $ 942
[1]
Number of common shares authorized (millions).1,1001,100
Number of common shares outstanding (millions).497509