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Losses and Loss Expense Reserve (Details 4) (USD $) In Thousands, unless otherwise specified | 6 Months Ended
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Jun. 30, 2012
Summary of rollforward of RMBS subrogation, by estimation approach |
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance | $ 2,720,266
Additional transactions reviewed, subrogation |
Additional adverse sample loans reviewed, subrogation |
Adverse loans repurchased by the sponsor, subrogation | 36
All other changes, subrogation | 49,889
Discounted RMBS subrogation (gross of reinsurance), Ending Balance | 2,770,191
Adverse Samples [Member] |
Summary of rollforward of RMBS subrogation, by estimation approach |
Discounted RMBS subrogation (gross of reinsurance), Number of Transaction, Beginning Balance | 30
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance | 1,457,472
Additional transactions reviewed, subrogation |
Additional adverse sample loans reviewed, subrogation |
Adverse loans repurchased by the sponsor, subrogation | 36
All other changes, subrogation | 95,687
All other changes, number of transaction | (1)
Discounted RMBS subrogation (gross of reinsurance), Ending Balance | 1,553,195
Random Samples [Member] |
Summary of rollforward of RMBS subrogation, by estimation approach |
Discounted RMBS subrogation (gross of reinsurance), Number of Transaction, Beginning Balance | 16
Discounted RMBS subrogation (gross of reinsurance), Beginning Balance | 1,262,794
Additional transactions reviewed, subrogation |
Additional adverse sample loans reviewed, subrogation |
Adverse loans repurchased by the sponsor, subrogation |
All other changes, subrogation | (45,798)
Discounted RMBS subrogation (gross of reinsurance), Ending Balance | $ 1,216,996

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