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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, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |
Sensitivity analysis, Net earnings increase (decrease) | CAD (12,000,000) | CAD (12,000,000) | CAD (1,000,000)
Sensitivity analysis, OCI gain | 0 | 0 | 5,000,000
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) | (5,000,000) | (5,000,000) | 2,000,000
Sensitivity analysis, OCI gain | 0 | 0 | 5,000,000
Australia, Dollars |  |  |
Disclosure of sensitivity analysis for actuarial assumptions [line items] |  |  |
Sensitivity analysis, Net earnings increase (decrease) | (7,000,000) | (7,000,000) | (3,000,000)
Sensitivity analysis, OCI gain | CAD 0 | CAD 0 | CAD 0

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