Income Taxes - Significant Portions of Deferred Tax Liabilities and Deferred Tax Assets (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]
Deferred tax liabilities:
Insurance intangible	$ 567,333
Variable interest entities	21,755	22,015
Deferred acquisition costs		57,262
Unearned premiums and credit fees	8,453	22,714
Unrealized gains on investments	12,823
Other	1,955	17,251
Total deferred tax liabilities	612,319	119,242
Deferred tax assets:
Unrealized losses & impairments on investments		12,605
Net operating loss and capital carryforward	2,110,938	2,857,926
Loss reserves	777,991	490,677
Compensation	8,030	7,184
Other	3,422	5,687
Sub-total deferred tax assets	2,900,381	3,374,079
Valuation allowance	2,289,673	3,256,423
Total deferred tax assets	610,708	117,656
Net deferred tax liability	$ (1,611)	$ (1,586)