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Foreign Exchange (Gain) Loss, Net - Schedule of Foreign Exchange Gain Loss Net (Detail) - CAD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Disclosure Of Effect Of Changes In Foreign Exchange Rates Gain Loss [Line Items]              
Unrealized Foreign Exchange (Gain) Loss $ 111 $ (140) $ (220) $ 229      
Realized Foreign Exchange (Gain) Loss 85 (19) 127 (61)      
Total 196 (159) (93) 168 $ (181) $ (404) $ 854
U.S. Dollar Debt Issued From Canada              
Disclosure Of Effect Of Changes In Foreign Exchange Rates Gain Loss [Line Items]              
Unrealized Foreign Exchange (Gain) Loss 148 (152) (132) 164      
Other              
Disclosure Of Effect Of Changes In Foreign Exchange Rates Gain Loss [Line Items]              
Unrealized Foreign Exchange (Gain) Loss $ (37) $ 12 $ (88) $ 65