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Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified | 8 Months Ended | 12 Months Ended | 4 Months Ended | 12 Months Ended
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Dec. 31, 2013 | Dec. 31, 2014 | Apr. 30, 2013 | Dec. 31, 2012
Successor [Member] |  |  |  |
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |  |
Balance at beginning | $ 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | $ 1,182 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |  |
Additions for credit impairments recognized on: |  |  |  |
Securities not previously impaired | 1,185 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 12,873 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |  |
Securities previously impaired |  | 7 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |  |
Reductions for credit impairments previously recognized on: |  |  |  |
Securities that matured or were sold during the period | (3)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |  |
Balance at end | 1,182 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember | 14,062 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember |  |
Predecessor [Member] |  |  |  |
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward] |  |  |  |
Balance at beginning |  |  | 183,300 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember | 201,317 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Additions for credit impairments recognized on: |  |  |  |
Securities not previously impaired |  |  |  | 824 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Securities previously impaired |  |  |  | 5,166 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Reductions for credit impairments previously recognized on: |  |  |  |
Securities that matured or were sold during the period |  |  | (183,300)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember | (24,007)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Balance at end |  |  | $ 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember | $ 183,300 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember

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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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