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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Credit Facilities (Details) CAD in Thousands, $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 CAD | Dec. 31, 2016 CAD | Dec. 31, 2016 USD ($) | Dec. 31, 2015 CAD | Dec. 31, 2015 USD ($) | Dec. 31, 2014 CAD
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Notional amount | CAD 3,664,000 | CAD 4,311,000 |  |  |  |
Borrowings | 3,707,000 | 4,361,000 |  |  |  |
Cash and cash equivalents | 314,000 | 305,000 |  | CAD 54,000 |  | CAD 43,000
Restricted cash (Note 21) | 30,000 | CAD 0 |  |  |  |
Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Increase (decrease) in borrowings | 500,000 |  |  |  |  |
Maximum borrowing capacity under line of credit | CAD 1,000,000 |  |  |  |  |
Term of credit facility | 4 years | 4 years |  |  |  |
Committed Bilateral Credit Facilities Expiring In 2019 [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | CAD 240,000 |  |  |  |  |
Committed Bilateral Credit Facilities In 2021 [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 200,000 |  |  |  |  |
Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 500,000 |  |  |  |  |
Credit Facility Cancelled In 2017 [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 350,000 |  |  |  |  |
Credit facilities | Committed bilateral credit facilities expiring in 2020 |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | $ |  |  | $ 200 |  | $ 300 |
Credit facilities | Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 2,000,000 | CAD 2,000,000 |  |  |  |
Undrawn borrowing capacity under line of credit | 1,400,000 | 1,400,000 |  |  |  |
Actual drawings on borrowings | 0 | 0 |  |  |  |
Long-term Line Of Credit | 600,000 | 600,000 |  |  |  |
Letter of credit | Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Long-term Line Of Credit | 600,000 | 600,000 |  |  |  |
Canadian Assets |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Net cash proceeds |  | 173,000 |  |  |  |
Corporation and TransAlta Renewables | Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 1,500,000 | CAD 1,500,000 |  |  |  |
TransAlta Renewables | Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | 500,000 |  |  |  |  |
TransAlta Renewables | Credit facilities | Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit | CAD 500,000 |  |  |  |  |

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