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Accounting Changes - Narrative (Details) - CAD CAD in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 01, 2018
Disclosure of changes in accounting estimates [line items] |  |  |  |
Depreciation and amortization | CAD 635 | CAD 601 | CAD 545 |
Alberta Mine | Change in Useful Lives Estimate |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |
Depreciation and amortization | 58 |  |  |
Sundance Unit 1 | Change in Useful Lives Estimate |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |
Depreciation and amortization | 26 |  |  |
Sundance Unit 2 | Change in Useful Lives Estimate |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |
Depreciation and amortization | CAD 4 |  |  |
IFRS 15 |  |  |  |
Disclosure of changes in accounting estimates [line items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption |  |  |  | CAD 12

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