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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Detail2) - USD ($) $ in Millions | Dec. 31, 2022 | Dec. 31, 2021
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Investment [Line Items] |  |
Securities fair value | $ 23 | $ 17
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 64 | 120
Standard & Poor's, B Rating |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 403 |
Standard & Poor's, B Rating | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 403 |
Standard & Poor's, B Rating | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 10 |
Standard & Poor's, B Rating | Municipal Bonds [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 10 |
Standard & Poor's, B Rating | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 0 |
Standard & Poor's, B Rating | Mortgage and Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 394 |
Standard & Poor's, B Rating | Mortgage and Asset-Backed Securities [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 394 |
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 2
Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 2
Standard & Poor's, BBB- Rating [Member] | Corporate Debt Securities [Member] | National Public Finance Guarantee Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 0
Standard & Poor's, C Rating | Assured Guaranty Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 1
Standard & Poor's, C Rating | Municipal Bonds [Member] | Assured Guaranty Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 1
Standard & Poor's, C Rating | Corporate Debt Securities [Member] | Assured Guaranty Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 0
Standard & Poor's, CCC+ Rating |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 757
Standard & Poor's, CCC+ Rating | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 754
Standard & Poor's, CCC+ Rating | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 318
Standard & Poor's, CCC+ Rating | Municipal Bonds [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 316
Standard & Poor's, CCC+ Rating | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 0
Standard & Poor's, CCC+ Rating | Mortgage and Asset-Backed Securities [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 439
Standard & Poor's, CCC+ Rating | Mortgage and Asset-Backed Securities [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | $ 439

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