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Credit Facilities, Long-Term Debt and Lease Liabilities - Non-Recourse Debt (Details)
$ in Millions, $ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2020
CAD ($)
Oct. 22, 2020
AUD ($)
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Jul. 20, 2018
Disclosure of detailed information about borrowings [line items]            
Interest rate     4.07%     4.509%
Borrowings   $ 3,361   $ 3,212    
Non-Recourse Variable Rate Debt | Minimum            
Disclosure of detailed information about borrowings [line items]            
Interest rate   2.95%        
Non-Recourse Variable Rate Debt | Maximum            
Disclosure of detailed information about borrowings [line items]            
Interest rate   4.51%        
Senior Secured Note Maturing June 2042            
Disclosure of detailed information about borrowings [line items]            
Interest rate     4.07%      
Borrowings     $ 800      
Non-Recourse Bonds            
Disclosure of detailed information about borrowings [line items]            
Interest rate         4.509%  
Redemption of noncurrent borrowings $ 25          
Non-Recourse Bonds | TransAlta OCP            
Disclosure of detailed information about borrowings [line items]            
Notional amount         $ 345