Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2014 Policies	Dec. 31, 2013 Policies	Sep. 30, 2013	Apr. 30, 2013
Less:
Claim liability reported on Balance Sheet, before reinsurance	$ 5,514,237,000	$ 5,470,234,000
Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	354	339
Remaining weighted-average contract period (in years)	15 years	14 years
Gross insured contractual payments outstanding:
Principal	20,458,621,000	22,206,789,000
Interest	7,835,469,000	8,397,133,000
Total	28,294,090,000	30,603,922,000
Gross undiscounted claim liability	10,511,657,000	11,276,259,000
Discount, gross claim liability	(1,784,862,000)	(2,452,448,000)
Gross claim liability before all subrogation and before reinsurance	8,726,795,000	8,823,811,000
Less:
Gross RMBS subrogation	(2,231,240,000)	(2,215,849,000)
Discount, RMBS subrogation	16,810,000	9,251,000
Discounted RMBS subrogation, before reinsurance	(2,214,430,000)	(2,206,598,000)	(2,361,856,000)	(2,482,918,000)
Less:
Gross other subrogation	(753,156,000)	(846,699,000)
Discount, other subrogation	85,262,000	91,313,000
Discounted other subrogation, before reinsurance	(667,894,000)	(755,386,000)
Gross claim liability, net of all subrogation and discounts, before reinsurance	5,844,471,000	5,861,827,000
Less: Unearned premium reserves	(441,797,000)	(502,984,000)
Plus: Loss adjustment expenses reserves	111,563,000	111,391,000
Claim liability reported on Balance Sheet, before reinsurance	5,514,237,000	5,470,234,000
Reinsurance recoverable reported on Balance Sheet	105,511,000	121,249,000
I/SL [Member] | Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	36	18
Remaining weighted-average contract period (in years)	8 years	13 years
Gross insured contractual payments outstanding:
Principal	898,817,000	834,708,000
Interest	344,788,000	506,903,000
Total	1,243,605,000	1,341,611,000
Gross undiscounted claim liability	16,134,000	7,447,000
Discount, gross claim liability	(1,124,000)	(1,225,000)
Gross claim liability before all subrogation and before reinsurance	15,010,000	6,222,000
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	15,010,000	6,222,000
Less: Unearned premium reserves	(10,604,000)	(4,060,000)
Plus: Loss adjustment expenses reserves	109,000
Claim liability reported on Balance Sheet, before reinsurance	4,515,000	2,162,000
Reinsurance recoverable reported on Balance Sheet	64,000	146,000
IA [Member] | Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	26	23
Remaining weighted-average contract period (in years)	12 years	19 years
Gross insured contractual payments outstanding:
Principal	478,130,000	1,125,284,000
Interest	208,929,000	871,751,000
Total	687,059,000	1,997,035,000
Gross undiscounted claim liability	11,620,000	54,398,000
Discount, gross claim liability	(1,043,000)	(6,726,000)
Gross claim liability before all subrogation and before reinsurance	10,577,000	47,672,000
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	10,577,000	47,672,000
Less: Unearned premium reserves	(3,441,000)	(22,901,000)
Plus: Loss adjustment expenses reserves	1,754,000	11,000
Claim liability reported on Balance Sheet, before reinsurance	8,890,000	24,782,000
Reinsurance recoverable reported on Balance Sheet	896,000	2,271,000
II [Member] | Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	41	52
Remaining weighted-average contract period (in years)	15 years	17 years
Gross insured contractual payments outstanding:
Principal	3,101,558,000	3,464,420,000
Interest	1,974,807,000	2,130,271,000
Total	5,076,365,000	5,594,691,000
Gross undiscounted claim liability	176,815,000	221,321,000
Discount, gross claim liability	(18,676,000)	(32,630,000)
Gross claim liability before all subrogation and before reinsurance	158,139,000	188,691,000
Less:
Gross other subrogation	(18,030,000)	(20,367,000)
Discount, other subrogation	7,035,000	9,522,000
Discounted other subrogation, before reinsurance	(10,995,000)	(10,845,000)
Gross claim liability, net of all subrogation and discounts, before reinsurance	147,144,000	177,846,000
Less: Unearned premium reserves	(76,255,000)	(95,550,000)
Plus: Loss adjustment expenses reserves	2,420,000	2,257,000
Claim liability reported on Balance Sheet, before reinsurance	73,309,000	84,553,000
Reinsurance recoverable reported on Balance Sheet	1,393,000	2,273,000
III [Member] | Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	86	76
Remaining weighted-average contract period (in years)	20 years	19 years
Gross insured contractual payments outstanding:
Principal	5,516,840,000	5,597,387,000
Interest	3,051,291,000	2,331,222,000
Total	8,568,131,000	7,928,609,000
Gross undiscounted claim liability	2,474,118,000	3,029,891,000
Discount, gross claim liability	(869,612,000)	(1,299,032,000)
Gross claim liability before all subrogation and before reinsurance	1,604,506,000	1,730,859,000
Less:
Gross RMBS subrogation	(6,000)	(4,516,000)
Discount, RMBS subrogation	0	15,000
Discounted RMBS subrogation, before reinsurance	(6,000)	(4,501,000)
Less:
Gross other subrogation	(112,998,000)	(116,145,000)
Discount, other subrogation	29,133,000	36,125,000
Discounted other subrogation, before reinsurance	(83,865,000)	(80,020,000)
Gross claim liability, net of all subrogation and discounts, before reinsurance	1,520,635,000	1,646,338,000
Less: Unearned premium reserves	(280,123,000)	(280,245,000)
Plus: Loss adjustment expenses reserves	9,152,000	1,658,000
Claim liability reported on Balance Sheet, before reinsurance	1,249,664,000	1,367,751,000
Reinsurance recoverable reported on Balance Sheet	112,683,000	119,795,000
IV [Member] | Successor Ambac [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies	164	169
Remaining weighted-average contract period (in years)	12 years	11 years
Gross insured contractual payments outstanding:
Principal	10,463,229,000	11,184,943,000
Interest	2,255,635,000	2,556,968,000
Total	12,718,864,000	13,741,911,000
Gross undiscounted claim liability	7,832,910,000	7,963,137,000
Discount, gross claim liability	(894,405,000)	(1,112,829,000)
Gross claim liability before all subrogation and before reinsurance	6,938,505,000	6,850,308,000
Less:
Gross RMBS subrogation	(2,231,234,000)	(2,211,333,000)
Discount, RMBS subrogation	16,810,000	9,236,000
Discounted RMBS subrogation, before reinsurance	(2,214,424,000)	(2,202,097,000)
Less:
Gross other subrogation	(622,128,000)	(710,187,000)
Discount, other subrogation	49,094,000	45,666,000
Discounted other subrogation, before reinsurance	(573,034,000)	(664,521,000)
Gross claim liability, net of all subrogation and discounts, before reinsurance	4,151,047,000	3,983,690,000
Less: Unearned premium reserves	(71,374,000)	(100,228,000)
Plus: Loss adjustment expenses reserves	98,128,000	107,465,000
Claim liability reported on Balance Sheet, before reinsurance	4,177,801,000	3,990,927,000
Reinsurance recoverable reported on Balance Sheet	(9,525,000)	(3,236,000)
V [Member] | Successor Ambac [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	47,000	47,000
Interest	19,000	18,000
Total	66,000	65,000
Gross undiscounted claim liability	60,000	65,000
Discount, gross claim liability	(2,000)	(6,000)
Gross claim liability before all subrogation and before reinsurance	58,000	59,000
Less:
Gross claim liability, net of all subrogation and discounts, before reinsurance	58,000	59,000
Claim liability reported on Balance Sheet, before reinsurance	$ 58,000	$ 59,000