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Significant Events - Acquisition of Two US Wind Projects (Details) $ in Millions, $ in Millions |  |  | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 02, 2019 CAD ($) | Jan. 02, 2019 USD ($) | Feb. 20, 2018 power_purchase_arrangement MW | Dec. 31, 2018 CAD ($) | Dec. 31, 2018 USD ($) | Apr. 20, 2018 CAD ($)
US Wind Projects [Member] |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Notes and debentures issued | $ |  |  |  |  |  | $ 25
US Wind Projects - Big Level [Member] |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Capacity of facility (in megawatts) | MW |  |  | 90 |  |  |
Term of power purchase arrangement |  |  | 15 years |  |  |
US Wind Projects - Antrim [Member] |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Capacity of facility (in megawatts) | MW |  |  | 29 |  |  |
Number of power purchase arrangements | power_purchase_arrangement |  |  | 2 |  |  |
Term of power purchase arrangement |  |  | 20 years |  |  |
TransAlta Renewables |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Payments for development project expenditure |  |  |  | $ 61 | $ 48 |
TransAlta Renewables | US Wind Projects [Member] |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Payments for development project expenditure | $ |  |  |  |  | $ 240 |
Funding Of Construction Costs [Member] | TransAlta Renewables |  |  |  |  |  |
Investment [Line Items] |  |  |  |  |  |
Payments for development project expenditure | $ 45 | $ 33 |  |  |  |

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