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Derivative Instruments - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Contract	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Value of obligation to return cash collateral, recorded in "Other liabilities"	$ 0		$ 0		$ 0
Net liability fair value of all derivative instruments linked to Ambac's own credit risk	58,935,000		58,935,000		79,912,000
Fair value of posted assets as collateral	$ 84,926,000		$ 84,926,000		111,391,000
Credit Derivatives [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Number of credit derivative contracts | Contract	0		0
Successor [Member]
Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]
Value of right to reclaim cash collateral and posted margin, recorded in "Other assets"	$ 10,888,000		$ 10,888,000		$ 20,926,000
Gains in change in fair value of the call options	353,777,000	$ (28,482,000)	606,464,000	$ 11,379,000
Gain (Loss) on Sale of Derivatives	$ 99,000	$ 134,000	$ 296,000	$ 1,467,000

X

- Definition

Number of Credit Derivative Contracts on adversely classified credit listing.

+ References

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Schedule Of Loans And Allowance For Loan By Class Individually And Collectively Evaluated For Impairment [Line Items]

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
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- Definition

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
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-Subparagraph (b)
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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
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-Paragraph 13
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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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-Publisher FASB
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- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

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-Publisher FASB
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-Topic 815
-SubTopic 10
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-Paragraph 4H
-Subparagraph (c)
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- Definition

The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings.

+ References

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-Publisher FASB
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