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Credit Facilities, Long-Term Debt and Lease Liabilities - Restricted Cash and Letters of Credit (Details) - CAD ($)
Dec. 31, 2021
Dec. 31, 2020
US Wind Projects - Big Level    
Disclosure of detailed information about borrowings [line items]    
Restricted cash (Note 22) $ 0 $ 9,000,000
Letter of credit    
Disclosure of detailed information about borrowings [line items]    
Exposure to credit risk on loan commitments and financial guarantee contracts 902,000,000 621,000,000
Amount exercised under letters of credit 0 0
TransAlta OCP    
Disclosure of detailed information about borrowings [line items]    
Restricted cash (Note 22) 17,000,000 17,000,000
TEC    
Disclosure of detailed information about borrowings [line items]    
Restricted cash (Note 22) 53,000,000 $ 45,000,000
Uncommitted demand letter facility    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 150,000,000  
Uncommitted demand letter facility | TransAlta Renewables Inc.    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 150,000,000  
Uncommitted Demand Letter Facility 2    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 100,000,000  
Bilateral Committed Credit Facilities    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit $ 240,000,000