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Significant Events - Sale of Economic Interest in Australian Assets to TransAlta Renewables Inc. (Details) - CAD ($) shares in Millions, $ in Millions |  |  | 1 Months Ended | 4 Months Ended | 7 Months Ended | 11 Months Ended | 12 Months Ended | 19 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 01, 2017 | Jul. 31, 2017 | Dec. 31, 2018 | Jul. 30, 2018 | Jan. 05, 2016 | Nov. 29, 2018 | Nov. 25, 2015 | Jun. 21, 2018 | Dec. 31, 2018 | May 06, 2015 | Jul. 31, 2017
Disclosure of detailed information about business combination [line items] |  |  |  |  |  |  |  |  |  |  |
Consideration received |  |  |  |  |  |  |  |  | $ (10) |  |
TransAlta Renewables |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about business combination [line items] |  |  |  |  |  |  |  |  |  |  |
Proportion of equity participation in subsidiary |  |  | 60.90% | 61.10% | 62.00% | 61.00% | 72.80% | 64.00% |  | 70.30% | 59.80%
TransAlta Renewables | Australian Assets |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about business combination [line items] |  |  |  |  |  |  |  |  |  |  |
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 | 26.1 |  |  |  |  |  |  |  |  |  |
Common shares | TransAlta Renewables | Australian Assets |  |  |  |  |  |  |  |  |  |  |
Disclosure of detailed information about business combination [line items] |  |  |  |  |  |  |  |  |  |  |
Equity securities received upon conversion of shares | 26.4 |  |  |  |  |  |  |  |  |  |

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