v3.25.4
Stock-based compensation (PSU Table) (Details) - CAD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2025
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Remittance of stock-based award participants' withholding tax obligation   $ 9 $ 52 $ 51
Share Units Plan | Equity Settled Awards | PSU-ROIC        
Roll forward of outstanding RSUs        
Units outstanding at beginning of period (in shares) 400,000 400,000    
Units granted (in shares)   300,000    
Units settled (in shares)   0    
Units forfeited (in shares)   (100,000)    
Units expired (in shares)   (200,000)    
Units outstanding at end of period (in shares)   400,000 400,000  
Weighted-average grant date fair value        
Weighted-average grant date fair value at beginning of period (in dollars per share) $ 123.77 $ 123.77    
Weighted-average grant date fair value of units granted in period (in dollars per share)   143.37    
Weighted-average grant date fair value of units settled in period (in dollars per share)   0    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 133.59    
Weighted-average grant date fair value of units expired in period (in dollars per share)   89.37    
Weighted-average grant date fair value at end of period (in dollars per share)   $ 153.18 $ 123.77  
Roll forward of nonvested RSUs        
Units nonvested at beginning of period (in shares) 400,000 400,000    
Units granted in period (in shares)   300,000    
Units vested during the year (in shares)   0    
Units forfeited in period (in shares)   (100,000)    
Units expired in period (in shares)   (200,000)    
Units nonvested at end of period (in shares)   400,000 400,000  
Weighted-average grant date fair value        
Beginning of period (in dollars per share) $ 123.77 $ 123.77    
Weighted-average grant date fair value of units granted in period (in dollars per share)   143.37    
Weighted-average grant date fair value of units vested in period (in dollars per share)   0    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 133.59    
Weighted-average grant date fair value of units expired in period (in dollars per share)   89.37    
End of period (in dollars per share)   $ 153.18 $ 123.77  
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Grant date fair value   $ 36    
Unrecognized compensation costs, awards other than options   $ 14    
Unrecognized compensation cost, period of recognition   1 year 10 months 24 days    
Units granted in period (in shares)   300,000    
Share Units Plan | Equity Settled Awards | PSU-ROIC | 2023 Grant Year        
Roll forward of nonvested RSUs        
Units granted in period (in shares)   200,000    
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Grant date fair value   $ 0    
Units granted in period (in shares)   200,000    
Share Units Plan | Equity Settled Awards | PSU-TSR        
Roll forward of outstanding RSUs        
Units outstanding at beginning of period (in shares) 400,000 400,000    
Units granted (in shares)   100,000    
Units settled (in shares)   (100,000)    
Units forfeited (in shares)   0    
Units expired (in shares)   (100,000)    
Units outstanding at end of period (in shares)   300,000 400,000  
Weighted-average grant date fair value        
Weighted-average grant date fair value at beginning of period (in dollars per share) $ 192.33 $ 192.33    
Weighted-average grant date fair value of units granted in period (in dollars per share)   186.96    
Weighted-average grant date fair value of units settled in period (in dollars per share)   179.03    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 191.49    
Weighted-average grant date fair value of units expired in period (in dollars per share)   174.04    
Weighted-average grant date fair value at end of period (in dollars per share)   $ 198.61 $ 192.33  
Roll forward of nonvested RSUs        
Units nonvested at beginning of period (in shares) 200,000 200,000    
Units granted in period (in shares)   100,000    
Units vested during the year (in shares)   0    
Units forfeited in period (in shares)   0    
Units expired in period (in shares)   (100,000)    
Units nonvested at end of period (in shares)   200,000 200,000  
Weighted-average grant date fair value        
Beginning of period (in dollars per share) $ 192.33 $ 192.33    
Weighted-average grant date fair value of units granted in period (in dollars per share)   186.96    
Weighted-average grant date fair value of units vested in period (in dollars per share)   0    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 191.49    
Weighted-average grant date fair value of units expired in period (in dollars per share)   174.04    
End of period (in dollars per share)   $ 198.61 $ 192.33  
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Grant date fair value   $ 30    
Unrecognized compensation costs, awards other than options   $ 22    
Unrecognized compensation cost, period of recognition   1 year 9 months 18 days    
Units granted in period (in shares)   100,000    
Share Units Plan | Equity Settled Awards | PSU-TSR | 2022 Grant Year        
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Performance vesting factor   50.00%    
Remittance of stock-based award participants' withholding tax obligation $ 4      
Share Units Plan | Equity Settled Awards | PSU-TSR | 2023 Grant Year        
Roll forward of nonvested RSUs        
Units granted in period (in shares)   100,000    
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Grant date fair value   $ 21    
Units granted in period (in shares)   100,000    
Share Units Plan | Equity Settled Awards | RSUs        
Roll forward of outstanding RSUs        
Units outstanding at beginning of period (in shares) 200,000 200,000    
Units granted (in shares)   300,000    
Units settled (in shares)   0    
Units forfeited (in shares)   0    
Units expired (in shares)   0    
Units outstanding at end of period (in shares)   500,000 200,000  
Weighted-average grant date fair value        
Weighted-average grant date fair value at beginning of period (in dollars per share) $ 157.55 $ 157.55    
Weighted-average grant date fair value of units granted in period (in dollars per share)   143.89    
Weighted-average grant date fair value of units settled in period (in dollars per share)   132.53    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 146.87    
Weighted-average grant date fair value of units expired in period (in dollars per share)   0    
Weighted-average grant date fair value at end of period (in dollars per share)   $ 148.51 $ 157.55  
Roll forward of nonvested RSUs        
Units nonvested at beginning of period (in shares) 200,000 200,000    
Units granted in period (in shares)   300,000    
Units vested during the year (in shares)   (100,000)    
Units forfeited in period (in shares)   0    
Units expired in period (in shares)   0    
Units nonvested at end of period (in shares)   400,000 200,000  
Weighted-average grant date fair value        
Beginning of period (in dollars per share) $ 157.55 $ 157.55    
Weighted-average grant date fair value of units granted in period (in dollars per share)   143.89    
Weighted-average grant date fair value of units vested in period (in dollars per share)   143.47    
Weighted-average grant date fair value of units forfeited in period (in dollars per share)   $ 146.87    
Weighted-average grant date fair value of units expired in period (in dollars per share)   0    
End of period (in dollars per share)   $ 149.18 $ 157.55  
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Grant date fair value   $ 49    
Unrecognized compensation costs, awards other than options   $ 27    
Unrecognized compensation cost, period of recognition   1 year 6 months    
Units granted in period (in shares)   300,000    
Share Units Plan | Equity Settled Awards | PSU-ROIC and PSU-TSR | 2023 Grant Year        
Share-based Compensation Arrangement by Share-based Payment Award, Additional General Disclosures [Abstract]        
Performance vesting factor   0.00%    
Share Units Plan | Equity Settled Awards | PSUs | 2023 Grant Year        
Roll forward of nonvested RSUs        
Units vested during the year (in shares)   0