Special Purpose Entities, Including Variable Interest Entities - Summary of Carrying Amount of Assets, Liabilities and Maximum Exposure to Loss of Ambac's Variable Interests in Non-Consolidated Variable Interest Entities (Detail) (Variable Interest Entity [Member], USD $) In Thousands, unless otherwise specified	Sep. 30, 2013 Successor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member]	Sep. 30, 2013 Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Global Structured Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Global Public Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Global Public Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Collateralized Debt Obligations [Member] Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Collateralized Debt Obligations [Member] Global Structured Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Residential Mortgage-Backed Securities [Member] Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Residential Mortgage-Backed Securities [Member] Global Structured Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Other Consumer Asset-Backed [Member] Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Other Consumer Asset-Backed [Member] Global Structured Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Other Commercial Asset-Backed [Member] Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Other Commercial Asset-Backed [Member] Global Structured Finance [Member] Predecessor Ambac [Member]	Sep. 30, 2013 Other [Member] Global Structured Finance [Member] Successor Ambac [Member]	Dec. 31, 2012 Other [Member] Global Structured Finance [Member] Predecessor Ambac [Member]	Dec. 31, 2012 Mortgage-Backed-Commercial [Member] Global Structured Finance [Member] Predecessor Ambac [Member]
Variable Interest Entities [Line Items]
Maximum Exposure To Loss	$ 77,171,746	$ 93,517,995	$ 41,303,781	$ 56,421,767	$ 35,867,965	$ 37,096,228	$ 3,868,207	$ 10,176,522	$ 19,777,790	$ 24,008,616	$ 5,508,153	$ 5,895,377	$ 7,717,747	$ 10,192,858	$ 4,431,884	$ 5,505,007	$ 643,387
Insurance Assets	1,692,484	1,840,265	1,173,272	1,298,086	519,212	542,179	4,955	9,673	580,630	603,867	72,534	101,494	401,277	451,048	113,876	132,004
Insurance Liabilities	6,616,639	7,463,635	6,020,293	6,830,277	596,346	633,358	8,263	13,328	3,924,594	3,969,336	959,853	1,042,522	527,750	1,215,074	599,833	590,017
Derivative Liabilities	$ 186,989	$ 201,434	$ 159,415	$ 172,771	$ 27,574	$ 28,663	$ 100,506	$ 113,057			$ 44,742	$ 45,610	$ 9,860	$ 7,293	$ 4,307	$ 4,393	$ 2,418