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Investments (Portion Of Unrealized Gains And Losses On Equity Investments Held) (Detail) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Gain (Loss) on Investments [Line Items] |  |  |  |
Realized investment gains (losses) |  |  | $ (23) | $ (37)
Equity and Trading securities [Member] | Net gains (losses) recognized during the period on equity and trading securities [Member] |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |
Realized investment gains (losses) | $ (1) | $ (3) | 2 | (31)
Equity and Trading securities [Member] | Net gains (losses) recognized during the period on equity and trading securities sold during the period [Member] |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |
Realized investment gains (losses) | (1) | (6) | (1) | (6)
Equity and Trading securities [Member] | Unrealized gains (losses) recognized during the period on equity and trading securities still held at the reporting date [Member] |  |  |  |
Gain (Loss) on Investments [Line Items] |  |  |  |
Realized investment gains (losses) | $ 0 | $ 3 | $ 3 | $ (25)

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