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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
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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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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