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Losses and Loss Expense Reserve - Summary of Rollforward of RMBS Subrogation, by Estimation Approach (Detail) (USD $) In Thousands, unless otherwise specified | 9 Months Ended
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Sep. 30, 2012 Transactions
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items] |
December 31, 2011 | $ 2,720,266
Additional transactions reviewed |
Additional adverse sample loans reviewed |
Adverse loans repurchased by the sponsor | 35
All other changes | (31,434)
Discounted RMBS subrogation (gross of reinsurance) at September 30, 2012 | 2,688,867
Number of transaction removed from the Adverse Sample subrogation population | 2
Random Samples [Member] |
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items] |
December 31, 2011 | 1,262,794
Additional transactions reviewed |
Additional adverse sample loans reviewed |
All other changes | (82,077)
Discounted RMBS subrogation (gross of reinsurance) at September 30, 2012 | 1,180,717
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2011 | 16
Discounted RMBS subrogation (gross of reinsurance) at September 30, 2012 | 16
Adverse Samples [Member] |
Summary Of Rollforward Of Subrogation By Estimation Approach [Line Items] |
December 31, 2011 | 1,457,472
Additional transactions reviewed |
Additional adverse sample loans reviewed |
Adverse loans repurchased by the sponsor | 35
All other changes | 50,643
Discounted RMBS subrogation (gross of reinsurance) at September 30, 2012 | $ 1,508,150
Discounted RMBS subrogation (gross of reinsurance) at December 31, 2011 | 30
All other changes | (2)
Discounted RMBS subrogation (gross of reinsurance) at September 30, 2012 | 28

X
- Definition This element represents the change in subrogation due to additional adverse sample loans reviewed. + References No definition available. + Details
Name: | abk_AdditionalAdverseSampleLoansReviewedChangeInSubrogation
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents the change in subrogation due to additional transactions reviewed. + References No definition available. + Details
Name: | abk_AdditionalTransactionsReviewedChangeInSubrogation
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This element represents the change in subrogation due to adverse loans repurchased by sponsor. + References No definition available. + Details
Name: | abk_AdverseLoansRepurchasedBySponsorChangeInSubrogation
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Number of total remediation transactions. + References No definition available. + Details
Name: | abk_NumberOfRemediationTransactions
Namespace Prefix: | abk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

X
- Definition Number of transaction removed from the Adverse Sample subrogation population. + References No definition available. + Details
Name: | abk_NumberOfTransactionRemovedFromAdverseSampleSubrogationPopulation
Namespace Prefix: | abk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Other changes number of transaction. + References No definition available. + Details
Name: | abk_OtherChangesNumberOfTransaction
Namespace Prefix: | abk_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Other changes subrogation. + References No definition available. + Details
Name: | abk_OtherChangesSubrogation
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

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