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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) - Successor [Member] - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Dec. 31, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]        
Balance at beginning $ 40,512 $ 17,179 $ 31,176 $ 14,062
Additions for credit impairments recognized on:        
Securities not previously impaired 580 1,660    
Securities previously impaired 8,756 1,457    
Reductions for credit impairments previously recognized on:        
Securities that matured or were sold during the period 0 0    
Balance at end $ 40,512 $ 17,179 $ 31,176 $ 14,062