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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