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Risk Management Activities - Sensitivity Analysis For Management Assessment (Details) - Currency risk - CAD ($) | 12 Months Ended
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |
Sensitivity analysis, Net earnings increase (decrease) | $ (24,000,000) | $ (20,000,000) | $ (12,000,000)
Sensitivity analysis, OCI gain | 2,000,000 | 0 | 0
Sensitivity analysis, management assessment | 0.04 | 0.04 | 0.04
U.S. Dollars |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |
Sensitivity analysis, Net earnings increase (decrease) | (18,000,000) | (13,000,000) | (5,000,000)
Sensitivity analysis, OCI gain | 2,000,000 | 0 | 0
Australia, Dollars |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |
Sensitivity analysis, Net earnings increase (decrease) | (6,000,000) | (7,000,000) | (7,000,000)
Sensitivity analysis, OCI gain | $ 0 | $ 0 | $ 0

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