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Significant Events - Changes in Internal Capitalization of U.S. Entities (Details) - CAD CAD in Millions |  | 12 Months Ended
--- | --- | ---
Dec. 15, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of detailed information about hedges [line items] |  |  |  |  |
Gains (losses) on hedges of net investments in foreign operations, net of tax | [1] |  | CAD 50 | CAD 18 | CAD (172)
U.S. Entities | Net investment hedges |  |  |  |  |
Disclosure of detailed information about hedges [line items] |  |  |  |  |
Reclassification of cumulative translation gains from OCI |  | CAD 10 |  |  |
Gains (losses) on hedges of net investments in foreign operations, net of tax |  | CAD 6 |  |  |
[1] | (5) Net of income tax expense of 2 for the year ended Dec. 31, 2017 (2016 - 5 expense, 2015 - 7 expense)

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