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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	5 Months Ended	4 Months Ended	9 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Sep. 30, 2012 Predecessor Ambac [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at beginning		$ 183,300	$ 201,317
Additions for credit impairments recognized on:
Securities not previously impaired	1,032		820
Securities previously impaired			4,933
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(3)	(183,300)	(24,007)
Securities that no longer have a portion of other-than-temporary impairment in other comprehensive income because of the company's intent to sell such securities
Balance at end	$ 1,029		$ 183,063

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Securities having No Longer Portion of Other than Temporary Impairment in Other Comprehensive Income, Credit Losses on Debt Securities Held For Sale

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No definition available.

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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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (e)

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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 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

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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

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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

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