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Non-Controlling Interests - Financial Position of Non-Controlling Interests (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Disclosure of subsidiaries [line items]      
Current assets $ 1,341 $ 1,312  
Current liabilities (1,117) (880)  
Total equity 4,062 4,134 $ 4,385
Non-controlling interests (Note 12) 1,101 1,137  
TransAlta Renewables      
Disclosure of subsidiaries [line items]      
Current assets 293 250  
Long-term assets 3,409 3,497  
Current liabilities (152) (159)  
Long-term liabilities (1,237) (1,192)  
Total equity (2,313) (2,396)  
Non-controlling interests (Note 12) $ (941) $ (961)  
Ownership and voting rights percentage 39.60% 39.10%  
TransAlta Cogeneration L.P.      
Disclosure of subsidiaries [line items]      
Current assets $ 41 $ 82  
Long-term assets 328 354  
Current liabilities (27) (54)  
Long-term liabilities (19) (28)  
Total equity (323) (354)  
Non-controlling interests (Note 12) $ (160) $ (176)  
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 5000.00%