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Capital Stock - Summary of Normal Course Issuer Bids (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 5,077,882 25,768,563
Cash amount $ 203 $ 1,289
2018 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 0 23,401,457
Cash amount $ 0 $ 1,159
2019 Bid [Member]    
Schedule Of Share Repurchase Programs [Line Items]    
Shares purchased 5,077,882 2,367,106
Cash amount $ 203 $ 130