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Employee Retirement Benefits - Additional Information (Detail) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |
Health care cost trend rate assumed for 2020 | 5.66% |  |
Health care cost trend rate assumed for 2040 | 3.57% |  |
Long-term expected return | 4.50% |  |
Actual rate of return | 8.10% | 6.60% |
Cost for defined contributions plans | $ 43 | $ 41 | $ 40
Plan assets invested in venture capital partnerships | 3.00% |  |
Cash contributions to pension plans in 2020 | $ 216 |  |
Post Retirement Benefit |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Sensitivity percentage | 1.00% |  |
Increase in service and interest cost | $ 5 |  |
Increase in Accumulated Postretirement Benefit Obligation | 75 |  |
Decrease in Service and interest cost | 4 |  |
Decrease in Accumulated Postretirement Benefit Obligation | $ 60 |  |
Equity Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target allocation securities | 30.00% |  |
Debt Securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Target allocation securities | 67.00% |  |

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