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Common Shares - The effects of the Corporation's purchase and cancellation of the common shares (Details) - CAD ($) $ / shares in Units, $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Disclosure of classes of share capital [line items] |  |  |
Deficit | $ (1,496) | $ (1,209) |
NCIB Program |  |  |
Disclosure of classes of share capital [line items] |  |  |
Number of shares issued and fully paid | 3,264,500 |  |
Par value per share | $ 7.02 |  |
Issued capital (Note 24) | $ 23 |  |
Weighted average share price | 35 |  |
Deficit | 12 |  |
Common shares |  |  |
Disclosure of classes of share capital [line items] |  |  |
Issued capital (Note 24) | $ 3,059 | $ 3,094 | $ 3,095

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