Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) (USD $) In Thousands, unless otherwise specified	5 Months Ended	4 Months Ended	12 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Beginning balance of loss and loss expense reserves, net of subrogation recoverable and reinsurance	$ 5,434,517	$ 5,974,731	$ 6,230,780
Current year:
Establishment of new loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	47,838	2,748	464,058
Claim and loss expense payments, net of subrogation and reinsurance	(163)	(58)	(20,765)
Establishment of RMBS subrogation recoveries, net of reinsurance	(315)	(159)
Total current year	47,360	2,531	443,293
Prior years:
Change in previously established loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	(322,165)	(52,642)	72,700
Claim and loss (payments) recoveries, net of subrogation and reinsurance	(2,376)	20,902	(944,860)
Change in previously established RMBS subrogation recoveries, net of reinsurance	119,294	(12,596)	172,818
Total prior years	(205,247)	(44,336)	(699,342)
Net change in loss and loss expense reserves	(157,887)	(41,805)	(256,049)
Net consolidation of certain VIE's		(498,409)
Ending loss and loss expense reserves, net of subrogation recoverable and reinsurance	$ 5,276,630	$ 5,434,517	$ 5,974,731