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Non-Controlling Interests - Financial Position of Non-Controlling Interests (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure of subsidiaries [line items]      
Current assets $ 1,902 $ 1,341  
Current liabilities (935) (1,117)  
Total equity 3,436 4,062 $ 4,134
Non-controlling interests 1,084 1,101  
TransAlta Renewables Inc.      
Disclosure of subsidiaries [line items]      
Current assets 743 293  
Long-term assets 2,913 3,409  
Current liabilities (364) (152)  
Long-term liabilities (987) (1,237)  
Total equity (2,305) (2,313)  
Non-controlling interests $ (948) $ (941)  
Ownership and voting rights percentage 39.90% 39.60%  
TransAlta Cogeneration, L.P.      
Disclosure of subsidiaries [line items]      
Current assets $ 69 $ 41  
Long-term assets 323 328  
Current liabilities (78) (27)  
Long-term liabilities (37) (19)  
Total equity (277) (323)  
Non-controlling interests $ (136) $ (160)  
Ownership and voting rights percentage 49.99% 49.99%  
TransAlta Cogeneration, L.P. | Coal facility      
Disclosure of subsidiaries [line items]      
Ownership and voting rights percentage 50.00%