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	9 Months Ended	4 Months Ended
	Sep. 30, 2013 Successor Ambac [Member]	Sep. 30, 2014 Successor Ambac [Member]	Apr. 30, 2013 Predecessor Ambac [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	$ 5,572,672	$ 5,470,234	$ 6,122,140
Less reinsurance on loss and loss expense reserves	138,155	122,357	147,409
Beginning balance of net loss and loss expense reserves	5,434,517	5,347,877	5,974,731
Current year:
Establishment of new loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	47,838	236	2,748
Claim and loss expense payments, net of subrogation and reinsurance	(163)	(4)	(58)
Establishment of RMBS subrogation recoveries, net of reinsurance	(315)	0	(159)
Total current year	47,360	232	2,531
Prior years:
Change in previously established loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	(322,165)	(934)	(52,642)
Claim and loss expense recoveries (payments), net of subrogation and reinsurance	(2,376)	67,736	20,902
Change in previously established RMBS subrogation recoveries, net of reinsurance	119,294	(6,315)	(12,596)
Total prior years	(205,247)	60,487	(44,336)
Net change in loss and loss expense reserves	(157,887)	60,719	(41,805)
Ending net loss and loss expense reserves	5,276,630	5,408,596	5,434,517
Add reinsurance on loss and loss expense reserves	123,425	105,641	138,155
Ending gross loss and loss expense reserves	5,400,055	5,514,237	5,572,672
Consolidation Of Variable Interest Entities On Loss And Loss Adjustment Expense Reserves	$ 0	$ 0	$ (498,409)