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Decommissioning and Other Provisions - Decommissioning and Restoration (Details) $ in Millions, $ in Millions | Dec. 31, 2018 CAD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 CAD ($) | Dec. 31, 2017 USD ($)
--- | --- | --- | --- | ---
Disclosure of detailed information about borrowings [line items] |  |  |  |
Undiscounted cash flows required for future removal of facilities | $ 1,000 |  |  |
Centralia Coal Mine | Surety Bonds |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Bonds issued |  | $ 139 |  | $ 139
Alberta Mine | Letters Of Credit [Member] |  |  |  |
Disclosure of detailed information about borrowings [line items] |  |  |  |
Bonds issued | $ 122 |  | $ 120 |

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