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Significant Events - Sale of TransAlta Renewables Shares to Alberta Investment Management Corporation (Details) - CAD CAD in Millions |  | 12 Months Ended
--- | --- | ---
Nov. 26, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of subsidiaries [line items] |  |  |  |
Notional amount |  | CAD 3,664 | CAD 4,311 |
Net proceeds on sale of non-controlling interest in subsidiary (Note 4) |  | 0 | 162 | CAD 404
Restricted cash (Note 21) |  | 30 | 0 |
TransAlta Renewables |  |  |  |
Disclosure of subsidiaries [line items] |  |  |  |
Common shares of subsidiary transferred | 20,512,820 |  |  |
Net proceeds on sale of non-controlling interest in subsidiary (Note 4) | CAD 200 |  |  |
Net proceeds from sale of ownership interest | CAD 193 |  |  |
Non-recourse |  |  |  |
Disclosure of subsidiaries [line items] |  |  |  |
Restricted cash (Note 21) |  | CAD 35 | 24 |
Non-recourse | Non-Recourse Bonds |  |  |  |
Disclosure of subsidiaries [line items] |  |  |  |
Redemption of noncurrent borrowings |  |  | CAD 10 |

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