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Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Impairments that were Recognized in Earnings on Securities Held (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Investments Debt And Equity Securities [Abstract]
Balance at beginning	$ 183,300	$ 201,317
Additions for credit impairments recognized on:
Securities not previously impaired		2,609
Securities previously impaired		462
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(9)
Balance at end	$ 183,291	$ 204,388

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This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis.

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This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized.

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The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity).

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This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period.

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