Obligations under Investment Agreements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Investments Agreements [Line Items]
Contractual interest rate of agreements, minimum	0.33%	0.40%
Contractual interest rate of agreements, maximum	6.04%	6.11%
Borrowings under investment agreements at principal less unamortized discount	$ 362,017	$ 546,546

Carrying value as of the balance sheet date of the sum of the obligations to contract holders to provide to them an agreed upon rate of return pursuant to the terms of the underlying contract and of securities that an institution sells and agrees to repurchase (the identical or substantially the same securities) as a seller-borrower at a specified date for a specified price, also known as a repurchase agreement, or repo. Guaranteed investment agreement contracts represent lending by the contract holders to the entity in return for a guaranteed (primarily fixed) interest rate until maturity, unless called earlier if the contracts provide that option to the contract holders (usually institutions). There is little or no insurance risk for the entity. Most repos involve obligations of the federal government ot its agencies, but other financial instruments, such as commercial paper, banker's acceptances, and negotiable certificates of deposit, are sometimes used in repos.

No definition available.

Name:	abk_BorrowingsUnderGuaranteedInvestmentAgreementsAndSecuritiesSoldUnderAgreementsToRepurchase
Namespace Prefix:	abk_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant