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	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning		$ 12,519	$ 14,062	$ 1,182
Additions for credit impairments recognized on:
Securities not previously impaired	$ 7,172	11,337
Securities previously impaired	4,059
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	$ 0	0
Balance at end		$ 12,519	$ 14,062	$ 1,182