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	3 Months Ended	12 Months Ended
Mar. 31, 2013 Policies	Dec. 31, 2012 Policies
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	305	316
Remaining weighted-average contract period (in years)	12 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 22,870,428	$ 24,060,370
Interest	8,034,857	9,243,183
Total	30,905,285	33,303,553
Gross undiscounted claim liability	12,115,351	12,120,462
Discount, gross claim liability	(2,281,736)	(2,138,631)
Gross claim liability before all subrogation and before reinsurance	9,833,615	9,981,831
Less:
Gross RMBS subrogation	(2,497,395)	(2,561,163)
Discount, RMBS subrogation	10,026	37,938
Discounted RMBS subrogation, before reinsurance	(2,487,369)	(2,523,225)
Less:
Gross other subrogation	(939,323)	(907,729)
Discount, other subrogation	65,915	36,949
Discounted other subrogation, before reinsurance	(873,408)	(870,780)
Gross claim liability, net of all subrogation and discounts, before reinsurance	6,472,838	6,587,826
Less: Unearned premium reserves	(563,471)	(603,794)
Plus: Loss adjustment expenses reserves	135,842	138,108
Claim liability reported on Balance Sheet, before reinsurance	6,045,209	6,122,140
Reinsurance recoverable reported on Balance Sheet	160,682	159,086
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	13	14
Remaining weighted-average contract period (in years)	13 years	15 years
Gross insured contractual payments outstanding:
Principal	375,901	311,157
Interest	217,054	166,276
Total	592,955	477,433
Gross undiscounted claim liability	6,442	2,135
Discount, gross claim liability	(525)	(219)
Gross claim liability before all subrogation and before reinsurance	5,917	1,916
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	5,917	1,916
Less: Unearned premium reserves	(3,931)	(1,179)
Claim liability reported on Balance Sheet, before reinsurance	1,986	737
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	19	12
Remaining weighted-average contract period (in years)	19 years	21 years
Gross insured contractual payments outstanding:
Principal	1,720,891	786,998
Interest	1,286,857	715,129
Total	3,007,748	1,502,127
Gross undiscounted claim liability	56,453	40,898
Discount, gross claim liability	(4,740)	(3,532)
Gross claim liability before all subrogation and before reinsurance	51,713	37,366
Less:
Gross RMBS subrogation	(93)
Discount, RMBS subrogation	1
Discounted RMBS subrogation, before reinsurance	(92)
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	51,621	37,366
Less: Unearned premium reserves	(30,958)	(21,626)
Plus: Loss adjustment expenses reserves	32
Claim liability reported on Balance Sheet, before reinsurance	20,695	15,740
Reinsurance recoverable reported on Balance Sheet	2,781	1,078
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	23	28
Remaining weighted-average contract period (in years)	20 years	18 years
Gross insured contractual payments outstanding:
Principal	1,204,275	1,245,793
Interest	535,433	379,237
Total	1,739,708	1,625,030
Gross undiscounted claim liability	42,156	49,521
Discount, gross claim liability	(8,465)	(3,247)
Gross claim liability before all subrogation and before reinsurance	33,691	46,274
Less:
Gross other subrogation	(20,043)
Discount, other subrogation	7,644
Discounted other subrogation, before reinsurance	(12,399)
Gross claim liability, net of all subrogation and discounts, before reinsurance	21,292	46,274
Less: Unearned premium reserves	(10,341)	(17,120)
Plus: Loss adjustment expenses reserves	3,300
Claim liability reported on Balance Sheet, before reinsurance	14,251	29,154
Reinsurance recoverable reported on Balance Sheet	2,465	7,085
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	100	114
Remaining weighted-average contract period (in years)	19 years	20 years
Gross insured contractual payments outstanding:
Principal	7,567,920	9,161,747
Interest	3,574,483	4,905,775
Total	11,142,403	14,067,522
Gross undiscounted claim liability	4,036,278	4,051,076
Discount, gross claim liability	(1,367,508)	(1,342,910)
Gross claim liability before all subrogation and before reinsurance	2,668,770	2,708,166
Less:
Gross RMBS subrogation	(6,417)	(16,170)
Discount, RMBS subrogation	18	312
Discounted RMBS subrogation, before reinsurance	(6,399)	(15,858)
Less:
Gross other subrogation	(139,834)	(141,012)
Discount, other subrogation	22,212	21,238
Discounted other subrogation, before reinsurance	(117,622)	(119,774)
Gross claim liability, net of all subrogation and discounts, before reinsurance	2,544,749	2,572,534
Less: Unearned premium reserves	(413,644)	(450,247)
Plus: Loss adjustment expenses reserves	5,593
Claim liability reported on Balance Sheet, before reinsurance	2,136,698	2,122,287
Reinsurance recoverable reported on Balance Sheet	137,915	128,333
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	149	147
Remaining weighted-average contract period (in years)	7 years	8 years
Gross insured contractual payments outstanding:
Principal	12,001,394	12,554,628
Interest	2,421,012	3,076,746
Total	14,422,406	15,631,374
Gross undiscounted claim liability	7,973,957	7,976,765
Discount, gross claim liability	(900,496)	(788,720)
Gross claim liability before all subrogation and before reinsurance	7,073,461	7,188,045
Less:
Gross RMBS subrogation	(2,490,885)	(2,544,993)
Discount, RMBS subrogation	10,007	37,626
Discounted RMBS subrogation, before reinsurance	(2,480,878)	(2,507,367)
Less:
Gross other subrogation	(779,446)	(766,717)
Discount, other subrogation	36,059	15,711
Discounted other subrogation, before reinsurance	(743,387)	(751,006)
Gross claim liability, net of all subrogation and discounts, before reinsurance	3,849,196	3,929,672
Less: Unearned premium reserves	(104,597)	(113,622)
Plus: Loss adjustment expenses reserves	126,917	138,108
Claim liability reported on Balance Sheet, before reinsurance	3,871,516	3,954,158
Reinsurance recoverable reported on Balance Sheet	17,521	22,590
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	1	1
Remaining weighted-average contract period (in years)	6 years	6 years
Gross insured contractual payments outstanding:
Principal	47	47
Interest	18	20
Total	65	67
Gross undiscounted claim liability	65	67
Discount, gross claim liability	(2)	(3)
Gross claim liability before all subrogation and before reinsurance	63	64
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	63	64
Claim liability reported on Balance Sheet, before reinsurance	$ 63	$ 64

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

3 Months Ended

12 Months Ended

Mar. 31, 2013

Policies

Dec. 31, 2012

Policies

Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]

Number of policies

305

316

Remaining weighted-average contract period (in years)

12 years