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Credit Facilities, Long-Term Debt, and Finance Lease Obligations - Credit Facilities (Details) $ in Millions, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2019 USD ($) | Dec. 31, 2019 CAD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2017 CAD ($) | Dec. 31, 2016 CAD ($)
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Line of credit facility, cancelled capacity |  |  | $ 200 |  |  |
Notional amount |  | $ 3,104 |  | $ 3,234 |  |
Borrowings |  | 3,212 |  | 3,267 |  |
Cash and cash equivalents |  | 411 |  | 89 | $ 314 | $ 305
Restricted cash |  | 32 |  | 66 |  |
US Wind Projects - Big Level |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Restricted cash (Note 22) |  | 15 |  |  |  |
Borrowing costs incurred |  | 122 |  |  |  |
Tax equity funding | $ 85 |  |  |  |  |
Proceeds from tax equity funding |  | 91 |  |  |  |
US Wind Projects - Antrim |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Tax equity funding | $ 41 |  |  |  |  |
Accelerated tax depreciation |  | 35 |  |  |  |
Lakeswind |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Borrowing costs incurred |  | 23 |  |  |  |
Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit |  | 1,300 |  |  |  |
Committed Bilateral Credit Facilities Expiring In 2019 [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit |  | 240 |  |  |  |
Credit facilities | Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit |  | 2,200 |  | 2,000 |  |
Undrawn borrowing capacity under line of credit |  | 1,300 |  | 900 |  |
Actual drawings on borrowings |  | 220 |  | 339 |  |
Long-term Line Of Credit |  | 900 |  | 1,100 |  |
Letter of credit | Committed credit facilities |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Long-term Line Of Credit |  | 690 |  | 720 |  |
Corporation and TransAlta Renewables | Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit |  | 2,000 |  | 1,800 |  |
Line of credit facility, increase (decrease) |  |  |  | 250 |  |
Line of credit facility, increase (decrease) |  |  |  | 250 |  |
TransAlta Renewables | Syndicated Bank Credit Facility [Member] |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Maximum borrowing capacity under line of credit |  | 700 |  | 500 |  |
Line of credit facility, increase (decrease) |  | 200 |  |  |  |
Line of credit facility, increase (decrease) |  | 200 |  |  |  |
TransAlta OCP |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |
Restricted cash (Note 22) |  | 17 |  | 35 |  |
Restricted cash and cash equivalents, principal amount |  | (10) |  |  |  |
Restricted cash |  | $ 10 |  | $ 27 |  |

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