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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) (Cash and Securities Pledged Directly from Investment Portfolio [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cash and Securities Pledged Directly from Investment Portfolio [Member]
Derivative [Line Items]
Fair Value of Cash and Underlying Securities	$ 646,663	$ 874,987
Fair Value of Cash and Securities Pledged to Investment and Repurchase Agreement Counterparties	375,412	564,036
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 271,251	$ 310,951

X

- Definition

Fair value of cash and securities pledged to derivative counterparties.

+ References

No definition available.

+ Details

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X

- Definition

Fair value of cash and securities pledged to investment and repurchase agreement counterparties.

+ References

No definition available.

+ Details

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X

- Definition

Total fair value of cash and underlying securities pledged.

+ References

No definition available.

+ Details

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Namespace Prefix:	abk_
Data Type:	xbrli:monetaryItemType
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X

- Details

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