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Significant Events - TransAlta Renewables Closes Offering of Common Shares (Details) $ in Millions |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 22, 2018 CAD ($) | Dec. 31, 2018 CAD ($) shares | Dec. 31, 2017 CAD ($) shares | Dec. 31, 2016 CAD ($) | Jun. 28, 2018 CAD ($) | Jun. 22, 2018 $ / shares shares | Mar. 17, 2016 shares
Disclosure of components of capital [Line Items] |  |  |  |  |  |  |
Issued preferred capital |  |  |  |  | $ 33 |  |
Number of shares issued | shares |  | 38,600,000 | 38,600,000 |  |  |  | 1,824,620.00
Net proceeds on sale of non-controlling interest in subsidiary (Note 4) |  | $ 144 | $ 0 | $ 162 |  |  |
TransAlta Renewables |  |  |  |  |  |  |
Disclosure of components of capital [Line Items] |  |  |  |  |  |  |
Number of shares issued | shares |  |  |  |  |  | 11,860,000 |
Par value per share | $ / shares |  |  |  |  |  | $ 12.65 |
Gross Proceeds From Changes In Ownership Interests In Subsidiaries That Do Not Result In Loss Of Control | $ 150 |  |  |  |  |  |
Net proceeds on sale of non-controlling interest in subsidiary (Note 4) | $ 144 |  |  |  |  |  |
Entity common stock, shares outstanding | shares |  |  |  |  |  | 161,000,000 |
Ownership interest (as a percent) | 61.00% |  |  |  |  |  |

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