Investments - Summary of Roll-Forward of Ambac's Cumulative Credit Impairments that were Recognized in Earnings on Securities Held (Detail) (USD $) In Thousands, unless otherwise specified	2 Months Ended	4 Months Ended	6 Months Ended
	Jun. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Jun. 30, 2012 Predecessor Ambac [Member]
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Balance at beginning		$ 183,300	$ 201,317
Additions for credit impairments recognized on:
Securities not previously impaired	2,002	467	466
Securities previously impaired			4,933
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period		(183,767)	(24,007)
Balance at end	$ 2,002		$ 182,709