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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Credit Facilities (Details)
$ in Millions, $ in Millions
 12 Months Ended
Dec. 31, 2018
CAD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
CAD ($)
Dec. 31, 2016
CAD ($)
Dec. 31, 2015
CAD ($)
Disclosure of detailed information about borrowings [line items]          
Line of credit facility, cancelled capacity   $ 200      
Notional amount $ 3,234   $ 3,664    
Borrowings 3,267   3,707    
Cash and cash equivalents 89   314 $ 305 $ 54
Restricted cash (Note 22) 0   30    
Restricted cash (Note 22) 66   0    
Syndicated Bank Credit Facility [Member]          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 1,250        
Committed Bilateral Credit Facilities Expiring In 2019 [Member]          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 240        
Committed credit facilities          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 500        
Credit Facility Cancelled In 2017 [Member]          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 350        
Credit facilities | Committed credit facilities          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 2,000   2,000    
Undrawn borrowing capacity under line of credit 900   1,400    
Actual drawings on borrowings 339   27    
Long-term Line Of Credit 1,100   627    
Letter of credit | Committed credit facilities          
Disclosure of detailed information about borrowings [line items]          
Long-term Line Of Credit 720   677    
Corporation and TransAlta Renewables | Syndicated Bank Credit Facility [Member]          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 1,750   1,500    
Line of credit facility, increase (decrease) 250        
TransAlta Renewables | Syndicated Bank Credit Facility [Member]          
Disclosure of detailed information about borrowings [line items]          
Maximum borrowing capacity under line of credit 500   500    
TransAlta OCP          
Disclosure of detailed information about borrowings [line items]          
Restricted cash (Note 22) 35   0    
Restricted cash (Note 22) $ 27   $ 0