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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Letters of Credit (Details) - CAD
Dec. 31, 2017
Dec. 31, 2016
Letter of credit    
Disclosure of detailed information about borrowings [line items]    
Exposure to credit risk on loan commitments and financial guarantee contracts CAD 677,000,000 CAD 566,000,000
Amount exercised under letters of credit 0 CAD 0
Committed bilateral credit facilities    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 240,000,000  
Uncommitted demand letter facility    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit 100,000,000  
Uncommitted demand letter facility | TransAlta Renewables    
Disclosure of detailed information about borrowings [line items]    
Maximum borrowing capacity under line of credit CAD 100,000,000