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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, 2019 | Jun. 30, 2018 | Jun. 30, 2019 | Jun. 30, 2018
Investments [Abstract] |  |  |  |
Proceeds from sales | $ 684 | $ 413 | $ 1,367 | $ 1,064
Available For Sale Securities Realized Gain Loss [Abstract] |  |  |  |
Gross realized gains | 16 | 1 | 21 | 3
Gross realized losses | $ (2) | $ (7) | $ (3) | $ (13)

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