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Share-Based Incentive Compensation Programs - Additional Information (Detail) - CAD ($) $ in Millions | 12 Months Ended
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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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