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	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 14,062	$ 1,182
Additions for credit impairments recognized on:
Securities not previously impaired	1,456	9,696
Securities previously impaired	2,282
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	0	0
Balance at end	$ 17,800	$ 10,878