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Non-Controlling Interests - Subsidiaries and Operations with Non-Controlling Interests (Details)
shares in Millions
 1 Months Ended 5 Months Ended 7 Months Ended 12 Months Ended 19 Months Ended
Aug. 01, 2017
shares
Jul. 31, 2017
Jan. 05, 2016
Dec. 31, 2017
Nov. 25, 2015
Dec. 31, 2017
Dec. 31, 2016
MW
May 06, 2015
Jul. 31, 2017
May 07, 2015
MW
TransAlta Cogeneration L.P.                    
Disclosure of subsidiaries [line items]                    
Proportion of ownership interests held by non-controlling interests           49.99% 49.99%      
TransAlta Renewables                    
Disclosure of subsidiaries [line items]                    
Proportion of ownership interests held by non-controlling interests           36.00% 40.20%      
Proportion of equity participation in subsidiary     62.00% 64.00% 72.80%     70.30% 59.80%  
TransAlta Renewables | Kent Hills wind farm                    
Disclosure of subsidiaries [line items]                    
Proportion of ownership interests held by non-controlling interests             17.00%      
Capacity of facility (in megawatts) | MW             150      
Australian Assets                    
Disclosure of subsidiaries [line items]                    
Capacity of facility (in megawatts) | MW                   575
TransAlta Renewables | Australian Assets                    
Disclosure of subsidiaries [line items]                    
Proportion of equity participation in subsidiary 64.00% 59.80%                
TransAlta Renewables | Class B shares | Australian Assets                    
Disclosure of subsidiaries [line items]                    
Shares offered for conversion | shares 26.1                  
TransAlta Renewables | Common shares | Australian Assets                    
Disclosure of subsidiaries [line items]                    
Equity securities received upon conversion of shares | shares 26.4