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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 1,253 |
Standard & Poor's, B- Rating [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 1,260 | $ 1,164
Standard & Poor's, B- Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 1,153
Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 6 | 11
Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 1 |
Municipal Bonds [Member] | Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 310 |
Municipal Bonds [Member] | Standard & Poor's, B- Rating [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 318 | 186
Municipal Bonds [Member] | Standard & Poor's, B- Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 176
Municipal Bonds [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 6 | 11
Municipal Bonds [Member] | Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 1 |
Corporate Debt Securities [Member] | Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 470 |
Corporate Debt Securities [Member] | Standard & Poor's, B- Rating [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 470 | 535
Corporate Debt Securities [Member] | Standard & Poor's, B- Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | 535
Corporate Debt Securities [Member] | Standard & Poor's, BBB- Rating [Member] | National Public Finance Guarantee Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 0 | 0
Corporate Debt Securities [Member] | Standard & Poor's, C Rating | Assured Guaranty Municipal Corporation |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 0 |
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, B Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | 473 |
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, B- Rating [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors | $ 473 | 442
Mortgage And Other Asset Backed Securities [Member] | Standard & Poor's, B- Rating [Member] | Ambac Assurance Corporation [Member] |  |
Schedule of Investments [Line Items] |  |
Fair value of securities that include benefit of guarantees provided by financial guarantors |  | $ 442

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