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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2020 | Jun. 30, 2019 | Jun. 30, 2020 | Jun. 30, 2019
Investment [Line Items] |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | $ 17 | $ 7 |  |
Trading Securities, Realized Gain (Loss) | 5 | 3 |  |
Investment expense | (1) | (1) | $ (3) | $ (3)
Securities available-for-sale and short-term | 26 | 78 | 57 | 125
Net investment income | 52 | 86 | 31 | 141
Gains (losses) on securities held as of reporting date [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities |  |  | (16) | 14
Trading Securities, Realized Gain (Loss) |  |  | (19) | 4
Other Investments Income | 12 | 4 | 4 | 10
Fixed Income Investments [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | 26 | 74 | 55 | 118
Short-Term [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | 1 | 5 | 4 | 10
Other Investments [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | $ 27 | $ 8 | $ (25) | $ 16

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