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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, 2021 | Dec. 31, 2020
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Investment [Line Items] |  |
Securities fair value | $ 17 | $ 8
Market Value of Secured Note Collateral | 0 | 178
Fair Value of Securities Held by Ambac UK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note | 669 |
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 120 | 140
Standard & Poor's, B Rating |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 757 |
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 | 754 |
Standard & Poor's, B Rating | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 318 |
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 | 316 |
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 | 439 |
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 | 439 |
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 | 6
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 | 6
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 | 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 |  | 1,273
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 |  | 1,266
Standard & Poor's, CCC+ Rating | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 327
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 |  | 320
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 |  | 465
Standard & Poor's, CCC+ Rating | Corporate Debt Securities [Member] | Ambac Assurance Corporation [Member] |  |
Investment [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 465
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 |  | 481
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 |  | $ 481
Standard & Poor's, A- 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, A- 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, A- 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 |

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- Definition Fair Value of Securities Held by Ambac UK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note + References No definition available. + Details
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