Losses and Loss Expense Reserve - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012 Policies	Dec. 31, 2011 Policies
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	312	330
Remaining weighted-average contract period (in years)	13 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 24,885,464	$ 31,072,226
Interest	9,363,469	13,963,359
Total	34,248,933	45,035,585
Gross undiscounted claim liability	12,610,749	12,511,194
Discount, gross claim liability	(2,074,770)	(2,279,109)
Gross claim liability before all subrogation and before reinsurance	10,535,979	10,232,085
Less:
Gross RMBS subrogation(1)	(2,717,594)	(2,778,645)
Discount, RMBS subrogation	28,727	58,379
Discounted RMBS subrogation, before reinsurance	(2,688,867)	(2,720,266)
Less:
Gross other subrogation(2)	(844,902)	(720,871)
Discount, other subrogation	26,592	52,539
Discounted other subrogation, before reinsurance	(818,310)	(668,332)
Gross claim liability, net of all subrogation and discounts, before reinsurance	7,028,802	6,843,487
Less: Unearned premium reserves	(648,402)	(546,520)
Plus: Loss adjustment expenses reserves	139,740	87,294
Claim liability reported on Balance Sheet, before reinsurance(3)	6,520,140	6,384,260
Reinsurance recoverable reported on Balance Sheet	172,439	159,902
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	26	27
Remaining weighted-average contract period (in years)	22 years	18 years
Gross insured contractual payments outstanding:
Principal	838,780	2,222,493
Interest	737,308	1,069,538
Total	1,576,088	3,292,031
Gross undiscounted claim liability	38,038	48,573
Discount, gross claim liability	(3,302)	(4,208)
Gross claim liability before all subrogation and before reinsurance	34,736	44,365
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	34,736	44,365
Less: Unearned premium reserves	(20,196)	(25,264)
Claim liability reported on Balance Sheet, before reinsurance(3)	14,540	19,101
Reinsurance recoverable reported on Balance Sheet	1,077	1,117
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	9	10
Remaining weighted-average contract period (in years)	9 years	10 years
Gross insured contractual payments outstanding:
Principal	156,012	354,713
Interest	85,878	100,448
Total	241,890	455,161
Gross undiscounted claim liability	4,381	10,667
Discount, gross claim liability	(189)	(2,316)
Gross claim liability before all subrogation and before reinsurance	4,192	8,351
Less:
Gross other subrogation(2)	(19)	(256)
Discount, other subrogation	5	77
Discounted other subrogation, before reinsurance	(14)	(179)
Gross claim liability, net of all subrogation and discounts, before reinsurance	4,178	8,172
Less: Unearned premium reserves	(1,801)	(5,126)
Claim liability reported on Balance Sheet, before reinsurance(3)	2,377	3,046
Reinsurance recoverable reported on Balance Sheet		9
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	26	38
Remaining weighted-average contract period (in years)	17 years	17 years
Gross insured contractual payments outstanding:
Principal	1,236,062	2,060,102
Interest	379,866	1,088,036
Total	1,615,928	3,148,138
Gross undiscounted claim liability	49,164	52,355
Discount, gross claim liability	(2,907)	(3,800)
Gross claim liability before all subrogation and before reinsurance	46,257	48,555
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	46,257	48,555
Less: Unearned premium reserves	(16,707)	(22,111)
Claim liability reported on Balance Sheet, before reinsurance(3)	29,550	26,444
Reinsurance recoverable reported on Balance Sheet	8,413	5,714
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	99	125
Remaining weighted-average contract period (in years)	21 years	19 years
Gross insured contractual payments outstanding:
Principal	9,498,608	13,342,814
Interest	5,209,411	8,117,496
Total	14,708,019	21,460,310
Gross undiscounted claim liability	4,180,640	4,766,815
Discount, gross claim liability	(1,351,849)	(1,440,704)
Gross claim liability before all subrogation and before reinsurance	2,828,791	3,326,111
Less:
Gross RMBS subrogation(1)	(19,404)	(461,725)
Discount, RMBS subrogation	356	12,278
Discounted RMBS subrogation, before reinsurance	(19,048)	(449,447)
Less:
Gross other subrogation(2)	(121,271)	(52,871)
Discount, other subrogation	14,285	6,550
Discounted other subrogation, before reinsurance	(106,986)	(46,321)
Gross claim liability, net of all subrogation and discounts, before reinsurance	2,702,757	2,830,343
Less: Unearned premium reserves	(473,814)	(335,332)
Claim liability reported on Balance Sheet, before reinsurance(3)	2,228,943	2,495,011
Reinsurance recoverable reported on Balance Sheet	145,556	139,499
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	151	129
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	13,155,955	13,092,057
Interest	2,950,986	3,587,812
Total	16,106,941	16,679,869
Gross undiscounted claim liability	8,338,457	7,632,709
Discount, gross claim liability	(716,506)	(828,061)
Gross claim liability before all subrogation and before reinsurance	7,621,951	6,804,648
Less:
Gross RMBS subrogation(1)	(2,698,190)	(2,316,920)
Discount, RMBS subrogation	28,371	46,101
Discounted RMBS subrogation, before reinsurance	(2,669,819)	(2,270,819)
Less:
Gross other subrogation(2)	(723,612)	(667,744)
Discount, other subrogation	12,302	45,912
Discounted other subrogation, before reinsurance	(711,310)	(621,832)
Gross claim liability, net of all subrogation and discounts, before reinsurance	4,240,822	3,911,997
Less: Unearned premium reserves	(135,884)	(158,687)
Plus: Loss adjustment expenses reserves	139,740	87,294
Claim liability reported on Balance Sheet, before reinsurance(3)	4,244,678	3,840,603
Reinsurance recoverable reported on Balance Sheet	17,393	13,563
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	1	1
Remaining weighted-average contract period (in years)	10 years	6 years
Gross insured contractual payments outstanding:
Principal	47	47
Interest	20	29
Total	67	76
Gross undiscounted claim liability	69	75
Discount, gross claim liability	(17)	(20)
Gross claim liability before all subrogation and before reinsurance	52	55
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	52	55
Claim liability reported on Balance Sheet, before reinsurance(3)	$ 52	$ 55