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Significant Events - Sale of Economic Interest in Australian Assets to TransAlta Renewables Inc. (Details)
 1 Months Ended 5 Months Ended 7 Months Ended 12 Months Ended 19 Months Ended
Aug. 01, 2017
shares
Jul. 31, 2017
May 07, 2015
CAD
shares
MW
Jan. 05, 2016
Dec. 31, 2017
Nov. 25, 2015
Dec. 31, 2017
CAD
May 06, 2015
Jul. 31, 2017
Disclosure of detailed information about business combination [line items]                  
Consideration received             CAD (10,000,000)    
Australian Assets                  
Disclosure of detailed information about business combination [line items]                  
Capacity of facility (in megawatts) | MW     575            
Number of operating assets     6            
Length of pipeline     270            
Consideration received     CAD 1,780,000,000            
TransAlta Renewables                  
Disclosure of detailed information about business combination [line items]                  
Proportion of equity participation in subsidiary       62.00% 64.00% 72.80%   70.30% 59.80%
TransAlta Renewables | Australian Assets                  
Disclosure of detailed information about business combination [line items]                  
Common shares issued (in shares) | shares     17,858,423            
Share price     CAD 12.65            
Net proceeds on issuance of preferred shares (Note 24)     226,000,000            
Share issue related cost     11,000,000            
Effect of share issuance cost on income cost     3,000,000            
Dividend-equivalent payments     1,000,000            
TransAlta Renewables | Australian Assets                  
Disclosure of detailed information about business combination [line items]                  
Cash consideration received     (211,000,000)            
Value of equity consideration received     CAD 1,067,000,000            
Proportion of equity participation in subsidiary 64.00% 59.80%              
Class B shares | TransAlta Renewables | Australian Assets                  
Disclosure of detailed information about business combination [line items]                  
Shares offered for conversion | shares 26,100,000                
Common shares | TransAlta Renewables | Australian Assets                  
Disclosure of detailed information about business combination [line items]                  
Equity securities received upon conversion of shares | shares 26,400,000