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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Dec. 31, 2019
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Investment [Line Items] |  |
Fair Value Of Securities Deposited With Governmental Authorities | $ 7 | $ 6
Fair Value of Securities Deposited in Connection wtih Letter of Credit | 1 |
Market Value of Secured Note Collateral | 181 | 197
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 152 | 85
Ambac Assurance [Member] |  |
Investment [Line Items] |  |
Unsettled Amounts Secured Notes from LSNI | 10 | 55
Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 1,260 | 1,164
Standard & Poor's, B- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 318 | 186
Standard & Poor's, B- Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 470 | 535
Standard & Poor's, B- Rating [Member] | Mortgage And Other Asset Backed Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 473 | 442
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 1,253 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 310 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 470 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] | Mortgage And Other Asset Backed Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 473 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value |  | 1,153
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value |  | 176
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value |  | 535
Ambac Assurance Corporation [Member] | Standard & Poor's, B- Rating [Member] | Mortgage And Other Asset Backed Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value |  | 442
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 6 | 11
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 6 | 11
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | $ 0 | $ 0

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