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Obligations under Investment Agreements - Principal Due under Investment and Investment Repurchase Agreements, Based on Expected Optional Draw Date (Detail) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012
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Investments [Line Items] |
Net payments due under investment agreements Principal Amount | $ 362,104
2013 [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount | 5,926
2014 [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount | 190,000
2015 [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount | 44,343
2016 [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount | 4,002
2017 [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount | 117,833
All later years [Member] |
Investments [Line Items] |
Net payments due under investment agreements Principal Amount |

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- Definition Amount of investment agreement principal due to be paid based on the expected optional draw date. + References No definition available. + Details
Name: | abk_InvestmentAgreementExpectedRepaymentsOfPrincipal
Namespace Prefix: | abk_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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