Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2019 USD ($) Policies	Dec. 31, 2018 USD ($) Policies	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 26,962	$ 22,623	$ 38,293	$ 40,658
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 253	$ 510	$ 778
Number of policies | Policies	233	231
Remaining weighted-average contract period (in years)	14 years	16 years
Gross insured contractual payments outstanding:
Principal	$ 6,956,772	$ 9,403,405
Interest	3,406,384	10,582,773
Total	10,363,156	19,986,178
Gross undiscounted claim liability	2,474,995	3,448,228
Discount, gross claim liability	572,500	1,091,958
Gross claim liability before all subrogation and before reinsurance	1,902,495	2,356,270
Less:
Gross RMBS subrogation	(1,792,616)	(1,809,937)
Discount, RMBS subrogation	30,399	39,391
Discounted RMBS subrogation, before reinsurance	(1,762,217)	(1,770,546)
Less:
Gross other subrogation	(705,060)	(784,891)
Discount, other subrogation	51,547	133,188
Discounted other subrogation, before reinsurance	(653,513)	(651,703)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(513,235)	(65,979)
Less: Unearned premium reserves	(60,413)	(106,662)
Plus: Loss adjustment expenses reserves	82,928	65,759
Claim liability reported on Balance Sheet, before reinsurance	(490,720)	(106,882)
Reinsurance recoverable reported on Balance Sheet	$ 27,215	$ 23,133
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	36	21
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 993,210	$ 916,530
Interest	489,600	487,702
Total	1,482,810	1,404,232
Gross undiscounted claim liability	6,295	4,019
Discount, gross claim liability	492	481
Gross claim liability before all subrogation and before reinsurance	5,803	3,538
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	5,803	3,538
Less: Unearned premium reserves	(3,695)	(943)
Plus: Loss adjustment expenses reserves	1,140	1,369
Claim liability reported on Balance Sheet, before reinsurance	3,248	3,964
Reinsurance recoverable reported on Balance Sheet	$ 186	$ 367
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	23	28
Remaining weighted-average contract period (in years)	18 years	19 years
Gross insured contractual payments outstanding:
Principal	$ 659,974	$ 708,249
Interest	590,727	631,708
Total	1,250,701	1,339,957
Gross undiscounted claim liability	53,913	63,712
Discount, gross claim liability	5,850	13,008
Gross claim liability before all subrogation and before reinsurance	48,063	50,704
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	(10,816)
Discount, other subrogation	0	7,318
Discounted other subrogation, before reinsurance	0	(3,498)
Gross claim liability, net of all subrogation and discounts, before reinsurance	48,063	47,206
Less: Unearned premium reserves	(9,463)	(10,073)
Plus: Loss adjustment expenses reserves	3,809	4,253
Claim liability reported on Balance Sheet, before reinsurance	42,409	41,386
Reinsurance recoverable reported on Balance Sheet	$ 9,225	$ 7,285
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	14	18
Remaining weighted-average contract period (in years)	8 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 201,537	$ 622,820
Interest	101,507	293,293
Total	303,044	916,113
Gross undiscounted claim liability	27,057	36,000
Discount, gross claim liability	1,627	3,069
Gross claim liability before all subrogation and before reinsurance	25,430	32,931
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	0
Discount, other subrogation	0	0
Discounted other subrogation, before reinsurance	0	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	25,430	32,931
Less: Unearned premium reserves	(1,870)	(5,085)
Plus: Loss adjustment expenses reserves	374	2,564
Claim liability reported on Balance Sheet, before reinsurance	23,934	30,410
Reinsurance recoverable reported on Balance Sheet	$ 3,673	$ 4,223
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	16	16
Remaining weighted-average contract period (in years)	18 years	22 years
Gross insured contractual payments outstanding:
Principal	$ 884,385	$ 1,705,464
Interest	422,528	6,979,130
Total	1,306,913	8,684,594
Gross undiscounted claim liability	558,922	992,019
Discount, gross claim liability	169,017	433,709
Gross claim liability before all subrogation and before reinsurance	389,905	558,310
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(43,231)	(136,541)
Discount, other subrogation	4,586	67,008
Discounted other subrogation, before reinsurance	(38,645)	(69,533)
Gross claim liability, net of all subrogation and discounts, before reinsurance	351,260	488,777
Less: Unearned premium reserves	(7,201)	(36,365)
Plus: Loss adjustment expenses reserves	1,361	(5,926)
Claim liability reported on Balance Sheet, before reinsurance	345,420	446,486
Reinsurance recoverable reported on Balance Sheet	$ 26,601	$ 26,096
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	141	145
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 4,179,704	$ 5,407,202
Interest	1,789,609	2,177,539
Total	5,969,313	7,584,741
Gross undiscounted claim liability	1,778,465	2,295,968
Discount, gross claim liability	392,990	637,548
Gross claim liability before all subrogation and before reinsurance	1,385,475	1,658,420
Less:
Gross RMBS subrogation	(1,792,616)	(1,809,937)
Discount, RMBS subrogation	30,399	39,391
Discounted RMBS subrogation, before reinsurance	(1,762,217)	(1,770,546)
Less:
Gross other subrogation	(649,071)	(624,654)
Discount, other subrogation	43,997	55,088
Discounted other subrogation, before reinsurance	(605,074)	(569,566)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(981,816)	(681,692)
Less: Unearned premium reserves	(38,005)	(53,987)
Plus: Loss adjustment expenses reserves	76,244	63,499
Claim liability reported on Balance Sheet, before reinsurance	(943,577)	(672,180)
Reinsurance recoverable reported on Balance Sheet	$ (12,470)	$ (14,838)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	3	3
Remaining weighted-average contract period (in years)	3 years	3 years
Gross insured contractual payments outstanding:
Principal	$ 37,962	$ 43,140
Interest	12,413	13,401
Total	50,375	56,541
Gross undiscounted claim liability	50,343	56,510
Discount, gross claim liability	2,524	4,143
Gross claim liability before all subrogation and before reinsurance	47,819	52,367
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(12,758)	(12,880)
Discount, other subrogation	2,964	3,774
Discounted other subrogation, before reinsurance	(9,794)	(9,106)
Gross claim liability, net of all subrogation and discounts, before reinsurance	38,025	43,261
Less: Unearned premium reserves	(179)	(209)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	37,846	43,052
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0