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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2021 | Sep. 30, 2020
Investment [Line Items] |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities | $ 0 | $ 3 |  |
Investment expense | (1) | (1) | $ (4) | $ (4)
Securities available-for-sale and short-term | 15 | 24 | 59 | 80
Net investment income | 21 | 37 | 112 | 69
Gains (losses) on securities held as of reporting date [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Net Realized and Unrealized Gain (Loss) on Trading Securities |  |  | 16 | (12)
Other Investments Income | (1) | 3 | 14 | 7
Fixed Income Investments [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | 16 | 24 | 63 | 79
Short-Term [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | 0 | 1 | 0 | 4
Other Investments [Member] |  |  |  |
Investment [Line Items] |  |  |  |
Gross Investment Income, Operating | $ 6 | $ 14 | $ 53 | $ (11)

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- Definition Amount, before investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 + Details
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