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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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