Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended		12 Months Ended			9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013 Successor Ambac [Member] Policies	Apr. 30, 2013 Successor Ambac [Member]	Dec. 31, 2012 Predecessor Ambac [Member] Policies	Apr. 30, 2013 Predecessor Ambac [Member]	Dec. 31, 2011 Predecessor Ambac [Member]	Sep. 30, 2013 I/SL [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 I/SL [Member] Predecessor Ambac [Member] Policies	Sep. 30, 2013 IA [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 IA [Member] Predecessor Ambac [Member] Policies	Sep. 30, 2013 II [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 II [Member] Predecessor Ambac [Member] Policies	Sep. 30, 2013 III [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 III [Member] Predecessor Ambac [Member] Policies	Sep. 30, 2013 IV [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 IV [Member] Predecessor Ambac [Member] Policies	Sep. 30, 2013 V [Member] Successor Ambac [Member] Policies	Dec. 31, 2012 V [Member] Predecessor Ambac [Member] Policies
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	309		316			18	14	43	12	27	28	76	114	144	147	1	1
Remaining weighted-average contract period (in years)	13 years		13 years			13 years	15 years	16 years	21 years	19 years	18 years	18 years	20 years	10 years	8 years	6 years	6 years
Gross insured contractual payments outstanding:
Principal	$ 22,316,449		$ 24,060,370			$ 849,629	$ 311,157	$ 2,185,961	$ 786,998	$ 2,112,936	$ 1,245,793	$ 5,676,583	$ 9,161,747	$ 11,491,293	$ 12,554,628	$ 47	$ 47
Interest	7,687,638		9,243,183			531,290	166,276	1,736,991	715,129	653,033	379,237	2,360,215	4,905,775	2,406,091	3,076,746	18	20
Total	30,004,087		33,303,553			1,380,919	477,433	3,922,952	1,502,127	2,765,969	1,625,030	8,036,798	14,067,522	13,897,384	15,631,374	65	67
Gross undiscounted claim liability	11,133,860		12,120,462			8,053	2,135	82,907	40,898	112,807	49,521	2,857,566	4,051,076	8,072,462	7,976,765	65	67
Discount, gross claim liability	(2,240,931)		(2,138,631)			(1,157)	(219)	(8,757)	(3,532)	(19,038)	(3,247)	(1,143,340)	(1,342,910)	(1,068,635)	(788,720)	(4)	(3)
Gross claim liability before all subrogation and before reinsurance	8,892,929		9,981,831			6,896	1,916	74,150	37,366	93,769	46,274	1,714,226	2,708,166	7,003,827	7,188,045	61	64
Less:
Gross RMBS subrogation	(2,371,244)		(2,561,163)									(5,139)	(16,170)	(2,366,105)	(2,544,993)
Discount, RMBS subrogation	9,388		37,938									14	312	9,374	37,626
Discounted RMBS subrogation, before reinsurance	(2,361,856)	(2,482,918)	(2,523,225)	(2,482,918)	(2,720,266)							(5,125)	(15,858)	(2,356,731)	(2,507,367)
Less:
Gross other subrogation	(842,504)		(907,729)							(17,834)		(106,799)	(141,012)	(717,871)	(766,717)
Discount, other subrogation	76,450		36,949							7,868		30,060	21,238	38,522	15,711
Discounted other subrogation, before reinsurance	(766,054)		(870,780)							(9,966)		(76,739)	(119,774)	(679,349)	(751,006)
Gross claim liability, net of all subrogation and discounts, before reinsurance	5,765,019		6,587,826			6,896	1,916	74,150	37,366	83,803	46,274	1,632,362	2,572,534	3,967,747	3,929,672	61	64
Less: Unearned premium reserves	(491,291)		(603,794)			(4,306)	(1,179)	(43,904)	(21,626)	(61,877)	(17,120)	(282,949)	(450,247)	(98,255)	(113,622)
Plus: Loss adjustment expenses reserves	126,327		138,108					86		1,778		2,469		121,994	138,108
Claim liability reported on Balance Sheet, before reinsurance	5,400,055		6,122,140			2,590	737	30,332	15,740	23,704	29,154	1,351,882	2,122,287	3,991,486	3,954,158	61	64
Reinsurance recoverable reported on Balance Sheet	$ 137,400		$ 159,086			$ 182		$ 2,699	$ 1,078	$ 2,742	$ 7,085	$ 118,493	$ 128,333	$ 13,284	$ 22,590