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Investments (Net Realized Gains (Losses) From Sales Of Available-For-Sale Securities) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Investments [Abstract] |  |  |
Proceeds from sales | $ 2,117 | $ 2,256 | $ 2,412
Available For Sale Securities Realized Gain Loss [Abstract] |  |  |
Gross realized gains | 6 | 40 | 84
Gross realized losses | $ (19) | $ (22) | $ (23)

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