v3.26.3
Common shares - Common Share Activities (Detail) - CAD ($)
shares in Thousands, $ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Common stock beginning balance, (in shares)     483,593    
Common stock, ending balance, (in shares) 472,012   472,012   483,593
Common stock beginning balance [1]     $ 895    
Common stock, ending balance [1] $ 874   $ 874   $ 895
Common Stock          
Increase (Decrease) in Stockholders' Equity [Roll Forward]          
Common stock beginning balance, (in shares)     483,593 509,045 509,045
Purchases at stated value, (in shares)     (11,581)   (25,452)
Common stock, ending balance, (in shares) 472,012   472,012   483,593
Common stock beginning balance     $ 895 $ 942 $ 942
Purchases at stated value $ (21) $ (23) (21) $ (23) (47)
Common stock, ending balance $ 874   $ 874   $ 895
[1]
Number of common shares authorized (millions).1,1001,100
Number of common shares outstanding (millions).472484