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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
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Impairment [Line Items]    
Balance at beginning $ 201,317 $ 139,766
Additions for credit impairments recognized on:    
Securities not previously impaired 824 57,446
Securities previously impaired 5,166 6,397
Reductions for credit impairments previously recognized on:    
Securities that matured or were sold during the period (24,007) (2,292)
Balance at end $ 183,300 $ 201,317