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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
	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	$ 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember	$ 1,182us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Additions for credit impairments recognized on:
Securities not previously impaired	1,185us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember	12,873us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Securities previously impaired		7us-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	0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Balance at end	1,182us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember	14,062us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_SuccessorMember
Predecessor [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning			183,300us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember	201,317us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Additions for credit impairments recognized on:
Securities not previously impaired				824us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember
Securities previously impaired				5,166us-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			$ 0us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember	$ 183,300us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld / us-gaap_StatementScenarioAxis = us-gaap_PredecessorMember

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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

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-SubTopic 10

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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)

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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

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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

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