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Credit Facilities, Long-Term Debt and Lease Liabilities - Credit facilities summarized (Details) - CAD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Disclosure of detailed information about borrowings [line items]    
Facility size $ 2,190  
Outstanding letters of credit 621  
Borrowings 3,361 $ 3,212
Available capacity 1,455  
Financial assets pledged as collateral for liabilities or contingent liabilities 0 3
TransAlta Renewables Inc.    
Disclosure of detailed information about borrowings [line items]    
Financial assets pledged as collateral for liabilities or contingent liabilities 49  
Revolving committed syndicated bank facility    
Disclosure of detailed information about borrowings [line items]    
Facility size 1,250  
Outstanding letters of credit 379  
Borrowings 114  
Available capacity 757  
Canadian committed bilateral credit facilities    
Disclosure of detailed information about borrowings [line items]    
Facility size 240  
Outstanding letters of credit 150  
Borrowings 0  
Available capacity 90  
Committed Credit Facility | TransAlta Renewables Inc.    
Disclosure of detailed information about borrowings [line items]    
Facility size 700  
Outstanding letters of credit 92  
Borrowings 0  
Available capacity 608  
Credit facilities    
Disclosure of detailed information about borrowings [line items]    
Borrowings 114 $ 220
Letter of credit    
Disclosure of detailed information about borrowings [line items]    
Debt instruments issued 89  
Letter of credit | TransAlta Renewables Inc.    
Disclosure of detailed information about borrowings [line items]    
Debt instruments issued 92  
Bilateral Credit Facility | Later than six months and not later than one year    
Disclosure of detailed information about borrowings [line items]    
Borrowings $ 80