Insurance [Line Items]

No definition available.

Name:	ambc_InsuranceLineItems
Namespace Prefix:	ambc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of financial guarantee premiums receivable related to insured transactions with non-investment grade internal ratings.

+ References

No definition available.

+ Details

Name:	ambc_PercentagePremiumsReceivableRelatedToTransactionsWithNonInvestmentGradeInternalRatings
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of Premiums Receivable Related to Non Investment Grade Structured Finance Transactions

+ References

No definition available.

+ Details

Name:	ambc_PercentageofPremiumsReceivableRelatedtoNonInvestmentGradeStructuredFinanceTransactions
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Past due premiums on policies insuring internally rated non-investment grade obligations.

+ References

No definition available.

+ Details

Name:	ambc_PremiumsPastDueOnNonInvestmentGradeObligation
Namespace Prefix:	ambc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) in the allowance for doubtful accounts.

+ References

No definition available.

+ Details

Name:	us-gaap_AllowanceForDoubtfulAccountsReceivablePeriodIncreaseDecrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the amount of premium revenue on financial guarantee insurance contracts recognized during the period that was accelerated for revenue recognition purposes.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6933581&loc=SL5749298-161291

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsAcceleratedPremiumRevenueAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of the premium receivable for financial guarantee insurance contracts reported in the statement of financial position. This element primarily would consist of those financial guarantee insurance contracts which premiums are to be collected over the contract period, but may, on occasion, include premiums which are to be collected in full at inception, but which straddle a financial reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(1),(5)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of cash inflow from premiums received for premium receivable on financial guarantee insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivableProceedsFromPremiumsReceived
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumReceivableRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accretion of the discount for premiums receivable on financial guarantee insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(4)(iii)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAccretionOfDiscountOnPremiumReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the premium receivable on financial guarantee insurance contracts from changes in the period of the contract.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(4)(i)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustmentChangeInPeriodOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) in the premiums receivable on financial guarantee insurance contracts from adjustments classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 50
-Paragraph 3
-Subparagraph (c)(4)(iv)
-URI http://asc.fasb.org/extlink&oid=6933075&loc=SL5748726-161290

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsPremiumsReceivableAdjustmentOtherAdjustments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-09)
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.5)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_PremiumsReceivableAllowanceForDoubtfulAccounts
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_SuccessorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CurrencyAxis=currency_GBP
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CurrencyAxis=currency_EUR
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Financial Guarantee Insurance Contracts - Summary of Gross Premium Receivable Roll-Forward (Direct and Assumed Contracts) (Detail) € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended						12 Months Ended
	Mar. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Mar. 31, 2017 EUR (€)	Mar. 31, 2017 USD ($)	Mar. 31, 2017 GBP (£)	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)
Successor [Member]
Insurance [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount		$ 9,392			$ 16,280
Percentage Premiums Receivable Related To Transactions With Non Investment Grade Internal Ratings	21.00%	21.00%	21.00%	22.00%	22.00%	22.00%
Percentage of Premiums Receivable Related to Non Investment Grade Structured Finance Transactions	14.00%	14.00%	14.00%				16.00%
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Beginning premium receivable		$ 586,312			$ 661,337
Premium receipts		15,381			17,978
Adjustments for changes in expected and contractual cash flows		(1,289)			1,352
Accretion of premium receivable discount		3,846			4,244
Uncollectable premiums		604			(12)
Other adjustments (including foreign exchange)		6,615			3,734
Ending premium receivable		580,707			652,677		$ 661,337
Premiums Receivable, Allowance for Doubtful Accounts		8,738						$ 9,331
Premiums Past Due On Non Investment Grade Obligation		500
United Kingdom, Pounds
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable		163,926	£ 116,815		185,204	£ 147,726
Euro Member Countries, Euro
Financial Guarantee Insurance Contracts, Premium Receivable [Roll Forward]
Ending premium receivable	€ 29,767	$ 36,679		€ 32,671	$ 34,908