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Losses and Loss Expense Reserve - Summary of Balance of Subrogation Recoveries and Related Claim Liabilities, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2012 Trial | Dec. 31, 2011 Trial
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Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 44 | 46
Gross claim liability before subrogation recoveries | $ 3,891,776 | $ 3,777,581
Subrogation recoveries(1) | (2,688,867) | (2,720,266)
Gross claim liability after subrogation recoveries | 1,202,909 | 1,057,315
Adverse Samples [Member] |  |
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 28 | 30
Gross claim liability before subrogation recoveries | 2,734,471 | 2,637,479
Subrogation recoveries(1) | (1,508,150) | (1,457,472)
Gross claim liability after subrogation recoveries | 1,226,321 | 1,180,007
Random Samples [Member] |  |
Schedule Of Balance Of Subrogation Recoveries And Related Claim Liabilities By Estimation Approach [Line Items] |  |
Count | 16 | 16
Gross claim liability before subrogation recoveries | 1,157,305 | 1,140,102
Subrogation recoveries(1) | (1,180,717) | (1,262,794)
Gross claim liability after subrogation recoveries | $ (23,412) | $ (122,692)

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- Definition Number of samples. + References No definition available. + Details
Name: | abk_NumberOfSamples
Namespace Prefix: | abk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: | abk_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This element represents the related gross claim liability net of subrogation recoveries for RMBS financial guarantee insurance contracts with credit deterioration that have estimated subrogation recoveries. + References No definition available. + Details
Name: | abk_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilitiesAfterSubrogationRecoveries
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the related gross claim liability for RMBS financial guarantee insurance contracts with credit deterioration that have estimated subrogation recoveries. + References No definition available. + Details
Name: | abk_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationGrossClaimLiabilitiesBeforeSubrogationRecoveries
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This element represents the reduction to gross claim liability for subrogation on insured RMBS financial obligations with credit deterioration. + References No definition available. + Details
Name: | abk_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationReductionsOfGrossClaimLiabilitySubrogation
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant