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, unless otherwise specified	5 Months Ended	9 Months Ended	4 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2014 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Balance at beginning	$ 0	$ 1,182	$ 183,300
Additions for credit impairments recognized on:
Securities not previously impaired	1,032	11,337	467
Securities previously impaired
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	(3)	0	(183,767)
Balance at end	$ 1,029	$ 12,519	$ 0