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Share Capital - Schedule of Paid in Surplus Includes Stock Based Compensation Expense (Detail) - CAD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of reserves within equity [line items]    
Beginning Balance $ 4,377 $ 4,367
Stock-Based Compensation Expense 14 10
Ending Balance 4,391 4,377
Pre-Arrangement Earnings [member]    
Disclosure of reserves within equity [line items]    
Beginning Balance 4,086 4,086
Stock-Based Compensation Expense 0 0
Ending Balance 4,086 4,086
Stock-Based Compensation [member]    
Disclosure of reserves within equity [line items]    
Beginning Balance 291 281
Stock-Based Compensation Expense 14 10
Ending Balance $ 305 $ 291