Financial Guarantee Insurance Contracts - Summary of Loss Reserve Roll-Forward, Net of Subrogation Recoverable and Reinsurance (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 4,789	$ (517)	$ 139
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Less reinsurance on loss and loss expense reserves			100,355
Prior years:
Add reinsurance on loss and loss expense reserves	74,519		100,355
Successor [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder Benefits and Claims Incurred, Ceded	(20,601)	(10,559)
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Beginning gross loss and loss expense reserves	3,798,733	5,470,234
Less reinsurance on loss and loss expense reserves	100,355	122,357
Beginning balance of net loss and loss expense reserves	3,698,378	5,347,877
Current year:
ambc_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsCurrentYear	1	151
Total current year	1	151
Prior years:
Change in previously established loss and loss expense reserves, gross of RMBS subrogation and net of reinsurance	(132,639)	(117,692)
Claim and loss expense (payments) recoveries, net of subrogation and reinsurance	104,202	9,375
Change in previously established RMBS subrogation recoveries, net of reinsurance	(44,195)	(18,331)
Liability For Unpaid Claims And Claims Adjustment Expense Change In Loss Reserves Prior Year	(281,036)	(145,398)
Net change in loss and loss expense reserves	(281,035)	(145,247)
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,417,343	5,202,630
Add reinsurance on loss and loss expense reserves	74,519	111,303
Ending gross loss and loss expense reserves	$ 3,491,862	$ 5,313,933