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Significant Events - Sale of TransAlta Renewables Shares to Alberta Investment Management Corporation (Details) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of subsidiaries [line items] |  |  |
Notional amount | $ 3,234 | $ 3,664 |
(Note 4) | 144 | 0 | $ 162
Restricted cash (Note 22) | 0 | 30 |
Non-recourse |  |  |
Disclosure of subsidiaries [line items] |  |  |
Restricted cash (Note 22) | $ 33 | $ 35 |

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