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Investments (Net Realized Gains (Losses) From Sales Of Available-For-Sale Securities) (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Investments [Abstract] |  |  |  |
Proceeds from sales | $ 218 | $ 290 | $ 396 | $ 569
Available For Sale Securities Realized Gain Loss [Abstract] |  |  |  |
Gross realized gains | 2 | 20 | 6 | 31
Gross realized losses | $ (3) | $ (4) | $ (8) | $ (12)

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