Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) (USD $) In Thousands, unless otherwise specified	8 Months Ended	12 Months Ended	4 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Apr. 30, 2013	Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ (1,108)	$ (517)
Prior years:
Add reinsurance on loss and loss expense reserves	122,357	100,355
Successor [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	(14,106)	(21,164)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	5,572,672	5,470,234
Less reinsurance on loss and loss expense reserves	138,155	122,357
Beginning balance of net loss and loss expense reserves	5,434,517	5,347,877
Current year:
ambc_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear	97,342	309
Claim and loss expense payments, net of subrogation and reinsurance	442	17
Establishment of RMBS subrogation recoveries, net of reinsurance	(315)	0
Total current year	96,585	292
Prior years:
Change in previously established loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	(514,728)	(269,606)
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	(59,184)	1,067,321
Change in previously established RMBS subrogation recoveries, net of reinsurance	272,319	(312,864)
Liability For Unpaid Claims And Claims Adjustment Expense Change In Loss Reserves Prior Year	(183,225)	(1,649,791)
Net change in loss and loss expense reserves	(86,640)	(1,649,499)
Consolidation Of Variable Interest Entities On Loss And Loss Adjustment Expense Reserves		0
Liability for Unpaid Claims and Claims Adjustment Expense, Net	5,347,877	3,698,378
Add reinsurance on loss and loss expense reserves	122,357	100,355
Ending gross loss and loss expense reserves	5,470,234	3,798,733
Predecessor [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered			1,879	675
Policyholder Benefits and Claims Incurred, Ceded			3,889	6,996
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves			6,122,140	6,384,260
Less reinsurance on loss and loss expense reserves			147,409	153,480
Beginning balance of net loss and loss expense reserves			5,974,731	6,230,780
Current year:
ambc_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear			2,748	464,058
Claim and loss expense payments, net of subrogation and reinsurance			58	20,765
Establishment of RMBS subrogation recoveries, net of reinsurance			(159)	0
Total current year			2,531	443,293
Prior years:
Change in previously established loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance			(52,642)	72,700
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance			(20,902)	944,860
Change in previously established RMBS subrogation recoveries, net of reinsurance			(12,596)	172,818
Liability For Unpaid Claims And Claims Adjustment Expense Change In Loss Reserves Prior Year			(44,336)	(699,342)
Net change in loss and loss expense reserves			(41,805)	(256,049)
Consolidation Of Variable Interest Entities On Loss And Loss Adjustment Expense Reserves	0		(498,409)	0
Liability for Unpaid Claims and Claims Adjustment Expense, Net			5,434,517	5,974,731
Add reinsurance on loss and loss expense reserves			138,155	147,409
Ending gross loss and loss expense reserves			$ 5,572,672	$ 6,122,140