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Share-Based Incentive Compensation Programs - Additional Information (Detail) - CAD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation expense charged against income	$ 34	$ 32	$ 14
Income tax benefit recognized in income related to compensation expense	9	9	4
Cash payment for compensation expense	50	$ 59	$ 71
Restricted Stock Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Before-tax unrecognized compensation expense related to non-vested restricted stock	$ 76
Weighted average vesting period of nonvested restricted stock	4 years 1 month 6 days
Restricted Stock Units | Restricted Stock Unit Plan 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage restricted stock units vested on third anniversary of the grant date	50.00%
Percentage restricted stock units vested on seventh anniversary of the grant date	50.00%
Restricted Stock Units | Restricted Stock Unit Plan 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage restricted stock units vested on fifth anniversary of the grant date	50.00%
Percentage restricted stock units vested on tenth anniversary of the grant date	50.00%
Restricted Stock Units | Restricted Stock Unit Plan 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage restricted stock units vested on fifth anniversary of the grant date	50.00%
Percentage restricted stock units vested on tenth anniversary of the grant date	50.00%

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Percentage of the restricted stock unites that are exercisable three years after the grant date (remainder are exercisable seven years after the grant date).

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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