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Significant Events - Other Narrative Disclosures (Details) $ in Millions, $ in Millions |  |  |  |  | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Aug. 02, 2018 CAD ($) | Nov. 01, 2017 USD ($) | Aug. 01, 2017 USD ($) | Nov. 24, 2016 CAD ($) | Dec. 31, 2018 CAD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) | Jul. 20, 2018 | Dec. 31, 2017 CAD ($)
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Borrowings |  |  |  |  | $ 3,267 |  |  |  | $ 3,707
Consideration received |  |  |  |  | 10 |  |  |  |
Debentures |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Interest rate | 6.40% |  |  |  |  |  |  | 6.40% |
Borrowings | $ 400 |  |  |  | $ 647 |  |  |  | 1,046
Redemption of unsecured debt | 425 |  |  |  |  |  |  |  |
Redemption of unsecured debt, premium | $ 19 |  |  |  |  |  |  |  |
6.65% Senior notes, due May 2018 |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Interest rate |  |  |  |  | 6.65% |  | 6.65% |  |
Borrowings |  |  |  |  | $ 500 |  | $ 500 |  |
Redemption of unsecured debt |  |  |  |  | 617 | $ 516 |  |  |
Redemption of unsecured debt, premium |  |  |  |  | 5 |  |  |  |
Power Purchase Arrangement |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Balancing Pool compensation estimate, termination of contractual arrangement |  |  |  |  | $ 56 |  |  |  | $ 157
Solomon Power Station | Fortescue Metals Group Limited |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Portion of consideration paid (received) consisting of cash and cash equivalents |  | $ (325) |  |  |  |  |  |  |
Consideration received |  |  | $ (335) |  |  |  |  |  |
Canadian Coal |  |  |  |  |  |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |  |  |  |  |  |
Government Agreements, Amount |  |  |  | $ 37 |  |  |  |  |

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