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Fair Value Measurements - Summary of Information about Described Model Inputs Used to Determine Fair Value of Each Class of Credit Derivatives (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule Of Information About Described Model Inputs Used To Determine Fair Value Of Each Class Of Credit Derivative [Line Items]
Notional outstanding	$ 5,861,060	$ 11,281,777
Other Contracts [Member]
Schedule Of Information About Described Model Inputs Used To Determine Fair Value Of Each Class Of Credit Derivative [Line Items]
Notional outstanding	1,170,257	1,424,623
Weighted average life (WAL) in years	7 years 2 months 12 days	7 years 10 months 24 days
Weighted average quotes	10.00%	10.00%
CVA percentage	21.00%	55.00%
Weighted average credit rating	A	A
Fair value of derivative liabilities	$ (86,110)	$ (62,854)

X

- Definition

Fair value input weighted average credit derivative quote as percentage of notional.

+ References

No definition available.

+ Details

Name:	ambc_FairValueInputWeightedAverageCreditDerivativeQuoteAsPercentageOfNotional
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Fair value input weighted average credit rating.

+ References

No definition available.

+ Details

Name:	ambc_FairValueInputWeightedAverageCreditRating
Namespace Prefix:	ambc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Fair value input weighted average life.

+ References

No definition available.

+ Details

Name:	ambc_FairValueInputWeightedAverageLife
Namespace Prefix:	ambc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Credit valuation adjustment (CVA) as a percentage of the credit derivative liability calculated excluding the CVA.

+ References

No definition available.

+ Details

Name:	ambc_PercentageOfCreditValueAdjustment
Namespace Prefix:	ambc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Details

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

X

- Definition

Fair value of the gross liabilities less the gross assets of a derivative liability or group of derivative liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 10

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13433-108611



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 15

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13495-108611



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958

+ Details

Name:	us-gaap_DerivativeLiabilityFairValueNet1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Aggregate notional amount of credit risk derivatives.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1A

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5579245-113959



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Notional Amount

-URI http://asc.fasb.org/extlink&oid=6519104



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 1B

-URI http://asc.fasb.org/extlink&oid=7476318&loc=SL5580258-113959

+ Details

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