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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, 2021 | Dec. 31, 2020
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Investment [Line Items] |  |
Fair Value Of Securities Deposited With Governmental Authorities | $ 9 | $ 8
Fair Value of Securities Deposited in Connection wtih Letter of Credit | 1 | 1
Market Value of Secured Note Collateral | 0 | 178
Fair Value of Securities Held by AmbacUK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note | 655 |
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 120 | 140
Ambac Assurance [Member] |  |
Investment [Line Items] |  |
Unsettled Amounts Secured Notes from LSNI | 0 | 9
Standard & Poor's, B Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 785 |
Standard & Poor's, B Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 320 |
Standard & Poor's, B Rating [Member] | Corporate Debt Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 0 |
Standard & Poor's, B Rating [Member] | Mortgage And Other Asset Backed Securities [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 466 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 783 |
Ambac Assurance Corporation [Member] | Standard & Poor's, B Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 317 |
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 | 466 |
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 2 | 6
National Public Finance Guarantee Corporation [Member] | Standard & Poor's, BBB- Rating [Member] | Municipal Bonds [Member] |  |
Investment [Line Items] |  |
Investment Owned, at Fair Value | 2 | 6
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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- Definition Fair Value of Securities Held by AmbacUK Where Capital Stock of AUK is Pledged as Collateral for Sitka AAC Note + References No definition available. + Details
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- Definition Value of the investment at close of period. For schedules of investments that are categorized, the value would be aggregated by category. For investment in and advances to affiliates, if operations of any controlled companies are different in character from those of the company, group such affiliates within divisions and by type of activities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 210 -Section 50 -Paragraph 6 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=99383244&loc=d3e12121-115841 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 5D -Subparagraph (SX 210.12-13D(Column C)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=SL120429264-123010 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611197-123010 + Details
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