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Per Share Amounts - Additional Information (Detail) - CAD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Earnings Per Share [Line Items]        
Dividends paid on common shares   $ 77 $ 260 $ 245
Common Share [Member]        
Earnings Per Share [Line Items]        
Dividends paid on common shares $ 77   $ 260 $ 245
Dividend paid per share $ 0.0625   $ 0.2125 $ 0.20
Dividend declared but not paid   $ 0.0175    
Dividends payable date   Mar. 31, 2021    
Dividends payables date of record   Mar. 15, 2021    
Preferred Share [Member]        
Earnings Per Share [Line Items]        
Dividends payable date   Mar. 31, 2021    
Dividend declared but not paid to preferred shareholders   $ 8    
NSRs [Member]        
Earnings Per Share [Line Items]        
Options excluded from diluted weighted average number of shares   31 32 34