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Change in Projected Benefit Obligation (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Projected benefit obligation at January 1	$ 8,359	$ 8,785
Current service cost	228	239	$ 217
Interest cost	324	302	313
Actuarial loss (gain)	1,053	(498)
Amendments	283
Benefits paid	(461)	(469)
Projected benefit obligation at December 31	9,786	8,359	8,785
Accumulated benefit obligation at December 31	8,814	7,661
Other postretirement benefits
Schedule Of Defined Benefit Plans Disclosures [Line Items]
Projected benefit obligation at January 1	582	670
Current service cost	16	17	16
Interest cost	20	22	23
Actuarial loss (gain)	99	(101)
Benefits paid	(24)	(26)
Projected benefit obligation at December 31	$ 693	$ 582	$ 670

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Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Amount of gain (loss) from change in actuarial assumptions which (increases) decreases benefit obligation of defined benefit plan. Assumptions include, but are not limited to, interest, mortality, employee turnover, salary, and temporary deviation from substantive plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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