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Credit Facilities, Long-Term Debt and Lease Liabilities - Letters of Credit (Details) - CAD ($)
Dec. 31, 2020
Dec. 31, 2019
Letter of credit    
Disclosure of detailed information about borrowings [line items]    
Exposure to credit risk on loan commitments and financial guarantee contracts $ 621,000,000 $ 690,000,000
Amount exercised under letters of credit 0 $ 0
Committed bilateral credit facilities    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 240,000,000  
Uncommitted demand letter facility    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 100,000,000  
Uncommitted demand letter facility | TransAlta Renewables Inc.    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit $ 100,000,000