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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

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