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Risk Management Activities - Foreign Currency Rate Risk Management (Details) - CAD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Financial Instruments [Abstract] |  |  |  |
Gains (losses) on derivatives designated as cash flow hedges, net of tax | [1] | $ 61 | $ (8) | $ 214
[1] | (5) Net of income tax expense of $16 million for the year ended Dec. 31, 2019 (2018 - $1 million recovery, 2017 - $77 million recovery).

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