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Fair Value Measurements - Information about Valuation Inputs for Fixed Income Securities Classified as Level 3 (Detail) | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2012 | Dec. 31, 2011
U.S. Agency Obligations [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Coupon rate | 0.00% | 6.91%
Maturity | 0 years | 1 year 3 months 29 days
Yield | 0.00% | 2.13%
Corporate Obligations [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Coupon rate | 0.35% | 0.60%
Maturity | 21 years 4 months 21 days | 21 years 5 months 9 days
Yield | 6.36% | 7.20%
Collateralized debt obligations [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Coupon rate | 1.06% | 0.86%
Maturity | 1 year 6 months | 1 year 6 months 18 days
Yield | 8.80% | 11.79%
Other commercial asset-backed securities [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Coupon rate | 0.82% | 1.41%
Maturity | 8 years 7 days | 3 years
Yield | 7.50% | 4.50%
European ABS transactions [Member] |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |
Coupon rate | 1.82% | 1.56%
Maturity | 12 years 7 months 2 days | 11 years 11 months 27 days
Yield | 4.39% | 3.90%

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- Definition Fair value inputs coupon rate. + References No definition available. + Details
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Period Type: | duration

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- Definition Fair value inputs maturity. + References No definition available. + Details
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Data Type: | xbrli:durationItemType
Balance Type: | na
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- Definition Fair value inputs yield. + References No definition available. + Details
Name: | abk_FairValueInputsYield
Namespace Prefix: | abk_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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