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Non-Controlling Interests - Financial Position of Non-Controlling Interests (Details) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of subsidiaries [line items]      
Current assets $ 1,312 $ 1,709  
Current liabilities 873 1,608  
Total equity 4,134 4,385 $ 4,663
Non-controlling interests (Note 12) 1,137 1,059  
TransAlta Renewables      
Disclosure of subsidiaries [line items]      
Current assets 250 145  
Long-term assets 3,497 3,483  
Current liabilities (159) (356)  
Long-term liabilities (1,192) (1,075)  
Total equity (2,396) (2,197)  
Non-controlling interests (Note 12) $ (961) $ (812)  
Ownership and voting rights percentage 39.10% 36.00%  
TransAlta Cogeneration L.P.      
Disclosure of subsidiaries [line items]      
Current assets $ 82 $ 193  
Long-term assets 354 404  
Current liabilities (54) (73)  
Long-term liabilities (28) (26)  
Total equity (354) (498)  
Non-controlling interests (Note 12) $ (176) $ (247)  
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%