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Common Shares - Issued and Outstanding (Details) - CAD ($)
shares in Millions, $ in Millions
 12 Months Ended
Oct. 05, 2023
Dec. 31, 2024
Dec. 31, 2023
Common shares      
Reversal of provision for repurchase of common shares under ASPP   $ (19) $ 19
Provision for repurchase of common shares under ASPP     (19)
Share buyback tax   2 $ 0
TransAlta Renewables Inc.      
Common shares      
Transaction costs $ 11    
TransAlta Renewables Inc. | Issued capital      
Common shares      
Transaction costs $ 4 $ 4  
Common shares      
Common shares      
Issued and outstanding, beginning of period (in shares)   306.9 268.1
Reversal (provision) for repurchase of common shares under ASPP (in shares)   1.7 0.0
Purchased and cancelled under the NCIB (in shares)   (13.5) (7.5)
Share-based payment plans (in shares)   0.8 0.8
Stock options exercised (in shares)   1.6 0.7
Issued for acquisition of TransAlta Renewables (in shares)   0.0 46.5
Provision for repurchase of common shares under ASPP (in shares)   0.0 (1.7)
Issued and outstanding, end of year, prior to ASPP (in shares)   297.5 306.9
Issued and outstanding, end of year   $ 3,285 $ 2,863
Reversal of provision for repurchase of common shares under ASPP   19 0
Purchased and cancelled under the NCIB   (146) (80)
Share-based payment plans   9 6
Stock options exercised   12 5
Issued for acquisition of TransAlta Renewables   0 510
Provision for repurchase of common shares under ASPP   0 (19)
Issued and outstanding, end of year   $ 3,179 $ 3,285
Common shares | Prior To Automatic Share Purchase Plan      
Common shares      
Issued and outstanding, beginning of period (in shares)   308.6  
Issued and outstanding, end of year, prior to ASPP (in shares)   297.5 308.6
Issued and outstanding, end of year   $ 3,304  
Issued and outstanding, end of year   $ 3,179 $ 3,304