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Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Losses on Debt Securities for Which Portion of Other-than-Temporary Impairment was Recognized in Other Comprehensive Income (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]        
Balance at beginning $ 67,085 $ 52,070 $ 31,176  
Additions for credit impairments recognized on:        
Securities not previously impaired 1,210 3,310 3,572  
Securities previously impaired 226 11,705 17,322  
Reductions for credit impairments previously recognized on:        
Securities that matured or were sold during the period   (56,067) 0 $ 0
Balance at end $ 12,454 $ 67,085 $ 52,070 $ 31,176