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	4 Months Ended	8 Months Ended	12 Months Ended
	Apr. 30, 2013	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Successor [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning		$ 0	$ 14,062	$ 1,182
Additions for credit impairments recognized on:
Securities not previously impaired		1,185	10,900	12,873
Securities previously impaired			6,214	7
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period		(3)	0	0
Balance at end	$ 0	1,182	$ 31,176	$ 14,062
Predecessor [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	183,300	$ 0
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(183,300)
Balance at end	$ 0