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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Letters of Credit (Details) - CAD ($) | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Letter of credit |  |
Disclosure of detailed information about borrowings [line items] |  |
Exposure to credit risk on loan commitments and financial guarantee contracts | $ 690,000,000 | $ 720,000,000
Amount exercised under letters of credit | 0 | $ 0
Committed bilateral credit facilities |  |
Disclosure of detailed information about borrowings [line items] |  |
Maximum borrowing capacity under line of credit | 240,000,000 |
Uncommitted demand letter facility |  |
Disclosure of detailed information about borrowings [line items] |  |
Maximum borrowing capacity under line of credit | 100,000,000 |
Uncommitted demand letter facility | TransAlta Renewables |  |
Disclosure of detailed information about borrowings [line items] |  |
Maximum borrowing capacity under line of credit | $ 100,000,000 |

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