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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Credit Facilities (Details)
CAD in Thousands, $ in Millions
 12 Months Ended
Dec. 31, 2017
CAD
Dec. 31, 2016
CAD
Dec. 31, 2016
USD ($)
Dec. 31, 2015
CAD
Dec. 31, 2015
USD ($)
Dec. 31, 2014
CAD
Disclosure of detailed information about borrowings [line items]            
Notional amount CAD 3,664,000 CAD 4,311,000        
Borrowings 3,707,000 4,361,000        
Cash and cash equivalents 314,000 305,000   CAD 54,000   CAD 43,000
Restricted cash (Note 21) 30,000 CAD 0        
Syndicated Bank Credit Facility [Member]            
Disclosure of detailed information about borrowings [line items]            
Increase (decrease) in borrowings 500,000          
Maximum borrowing capacity under line of credit CAD 1,000,000          
Term of credit facility 4 years 4 years        
Committed Bilateral Credit Facilities Expiring In 2019 [Member]            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit CAD 240,000          
Committed Bilateral Credit Facilities In 2021 [Member]            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 200,000          
Committed credit facilities            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 500,000          
Credit Facility Cancelled In 2017 [Member]            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 350,000          
Credit facilities | Committed bilateral credit facilities expiring in 2020            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit | $     $ 200   $ 300  
Credit facilities | Committed credit facilities            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 2,000,000 CAD 2,000,000        
Undrawn borrowing capacity under line of credit 1,400,000 1,400,000        
Actual drawings on borrowings 0 0        
Long-term Line Of Credit 600,000 600,000        
Letter of credit | Committed credit facilities            
Disclosure of detailed information about borrowings [line items]            
Long-term Line Of Credit 600,000 600,000        
Canadian Assets            
Disclosure of detailed information about borrowings [line items]            
Net cash proceeds   173,000        
Corporation and TransAlta Renewables | Syndicated Bank Credit Facility [Member]            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 1,500,000 CAD 1,500,000        
TransAlta Renewables | Syndicated Bank Credit Facility [Member]            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit 500,000          
TransAlta Renewables | Credit facilities | Committed credit facilities            
Disclosure of detailed information about borrowings [line items]            
Maximum borrowing capacity under line of credit CAD 500,000