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Subsequent Event - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified | 12 Months Ended | 1 Months Ended |
--- | --- | --- | ---
Dec. 31, 2012 | Apr. 30, 2013 Subsequent Event [Member] | Dec. 31, 2012 Asset-backed Securities [Member]
Subsequent Event [Line Items] |  |  |
Loss on investment | $ 150,000 |  |
Investment in asset-backed notes |  |  | 174,500
Cash recoveries expected |  | $ 40,000 |

X
- Definition This item represents the cumulative total other than temporary impairment loss included in earnings relating to the subject investments. + References No definition available. + Details
Name: | abk_CumulativeOtherThanTemporaryImpairment
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Amount invested in an individual debt security. + References No definition available. + Details
Name: | abk_InvestmentSecurityPurchaseAmount
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition The cash inflow from recoveries that were previously other than temporarily impaired and deemed uncollectible. + References No definition available. + Details
Name: | abk_ProceedsFromRecoveriesOfInvestmentPreviouslyImpaired
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration