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Significant Events - TransAlta Renewables Project Financing of New Brunswick (Details) CAD in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Oct. 12, 2017 CAD | Oct. 02, 2017 CAD | Jun. 30, 2017 MW | Dec. 31, 2017 CAD | Dec. 31, 2016 CAD
Investment [Line Items] |  |  |  |  |
Notional amount |  |  |  | CAD 3,664 | CAD 4,311
Interest rate |  |  |  | 6.65% | 6.65%
Kent Hills Wind L.P. |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Interest rate |  |  |  | 4.55% |
Proportion of ownership interests held by non-controlling interests |  |  |  | 17.00% |
Kent Hills Wind L.P. | Kent Hills 3 Wind Project |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Increase in contractual power capacity (in megawatts) | MW |  |  | 17.25 |  |
Kent Hills Wind L.P. | Kent Hills 3 Wind Project | Natural Forces Technologies Inc. |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Proportion of ownership interests held by non-controlling interests |  | 17.00% |  |  |
Bond, 4.454%, November 2033 maturity | Kent Hills Wind L.P. |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Notional amount |  | CAD 260 |  |  |
Interest rate |  | 4.454% |  |  |
Proceeds from non-current borrowings |  | CAD 30 |  |  |
Unsecured debentures |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Notional amount |  |  |  | CAD 1,051 | CAD 1,051
Unsecured debentures | Canadian Hydro Developers Inc. |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Redemption of unsecured debt | CAD 201 |  |  |  |
Redemption of unsecured debt, principal |  |  |  | CAD 191 |
Redemption of unsecured debt, premium | 6 |  |  |  |
Redemption of unsecured debt, accrued interest | CAD 4 |  |  |  |

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