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Commitments and Contingencies (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 21, 2020
CAD ($)
Oct. 22, 2020
CAD ($)
Dec. 31, 2020
CAD ($)
J
Dec. 31, 2019
CAD ($)
Dec. 31, 2018
CAD ($)
Dec. 31, 2020
CAD ($)
Dec. 31, 2020
terajoule
Dec. 31, 2020
USD ($)
Jul. 30, 2015
USD ($)
Disclosure of Commitments [Line Items]                  
Transportation agreement, term     15 years            
Long-term firm natural gas delivery transportation, amount transferred per day     400       400    
Minimum operating lease payments recognised as expense         $ 8        
Sublease payments received (amounts less than)     $ 2 $ 1 $ 1        
TransAlta Energy Transition Bill           $ 18     $ 55
Funded portion of energy bill commitment               $ 41  
Total payments, line loss proceedings $ 11 $ 6 25            
Accrued payments, line loss proceedings     $ 8            
Estimated financial effect of contingent liabilities           12      
Emissions performance credits allotted, per year, amount           17      
Loss contingency accrual           22      
Increase (decrease) in loss contingency accrual, percentage     15.00%            
Increase (decrease) in loss contingency accrual           3      
AltaLink Management Ltd. [Member]                  
Disclosure of Commitments [Line Items]                  
Loss contingency accrual           $ 91      
NOVA Gas Transmission Ltd.                  
Disclosure of Commitments [Line Items]                  
Long-term firm natural gas delivery transportation, amount transferred per day     351       275    
Tidewater                  
Disclosure of Commitments [Line Items]                  
Long-term firm natural gas delivery transportation, amount transferred per day     139       139