Financial Guarantee Insurance Contracts - Summary of Information Related to Policies Currently Included in Ambac's Loss Reserves or Subrogation Recoverable (Detail) - Successor [Member] $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018 USD ($) Policies	Dec. 31, 2017 USD ($) Policies	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 38,293	$ 40,658	$ 47,034	$ 30,767
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 778	$ 339	$ (568)
Number of policies | Policies	240	277
Remaining weighted-average contract period (in years)	16 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 9,994,954	$ 11,687,070
Interest	10,805,030	11,197,612
Total	20,799,984	22,884,682
Gross undiscounted claim liability	4,042,471	8,025,561
Discount, gross claim liability	1,367,007	1,291,005
Gross claim liability before all subrogation and before reinsurance	2,675,464	6,734,556
Less:
Gross RMBS subrogation	(1,851,859)	(1,857,502)
Discount, RMBS subrogation	35,474	23,115
Discounted RMBS subrogation, before reinsurance	(1,816,385)	(1,834,387)
Less:
Gross other subrogation	(712,785)	(827,078)
Discount, other subrogation	81,900	86,822
Discounted other subrogation, before reinsurance	(630,885)	(740,256)
Gross claim liability, net of all subrogation and discounts, before reinsurance	228,194	4,159,913
Less: Unearned premium reserves	(121,401)	(135,502)
Plus: Loss adjustment expenses reserves	74,353	89,391
Claim liability reported on Balance Sheet, before reinsurance	181,146	4,113,802
Reinsurance recoverable reported on Balance Sheet	$ 39,071	$ 40,997
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	29	26
Remaining weighted-average contract period (in years)	10 years	10 years
Gross insured contractual payments outstanding:
Principal	$ 968,349	$ 1,046,267
Interest	523,712	531,657
Total	1,492,061	1,577,924
Gross undiscounted claim liability	4,231	4,434
Discount, gross claim liability	534	465
Gross claim liability before all subrogation and before reinsurance	3,697	3,969
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	3,697	3,969
Less: Unearned premium reserves	(1,425)	(2,126)
Plus: Loss adjustment expenses reserves	2,778	16,116
Claim liability reported on Balance Sheet, before reinsurance	5,050	17,959
Reinsurance recoverable reported on Balance Sheet	$ 237	$ 202
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	26	20
Remaining weighted-average contract period (in years)	22 years	23 years
Gross insured contractual payments outstanding:
Principal	$ 571,525	$ 531,190
Interest	578,171	584,098
Total	1,149,696	1,115,288
Gross undiscounted claim liability	56,137	56,659
Discount, gross claim liability	14,068	13,095
Gross claim liability before all subrogation and before reinsurance	42,069	43,564
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(8,855)	(7,990)
Discount, other subrogation	5,982	5,169
Discounted other subrogation, before reinsurance	(2,873)	(2,821)
Gross claim liability, net of all subrogation and discounts, before reinsurance	39,196	40,743
Less: Unearned premium reserves	(9,951)	(9,990)
Plus: Loss adjustment expenses reserves	1,419	3,242
Claim liability reported on Balance Sheet, before reinsurance	30,664	33,995
Reinsurance recoverable reported on Balance Sheet	$ 4,404	$ 4,894
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	13	26
Remaining weighted-average contract period (in years)	9 years	10 years
Gross insured contractual payments outstanding:
Principal	$ 583,420	$ 1,199,909
Interest	228,073	413,045
Total	811,493	1,612,954
Gross undiscounted claim liability	48,815	77,289
Discount, gross claim liability	6,569	12,250
Gross claim liability before all subrogation and before reinsurance	42,246	65,039
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	0	(9,371)
Discount, other subrogation	0	2,550
Discounted other subrogation, before reinsurance	0	(6,821)
Gross claim liability, net of all subrogation and discounts, before reinsurance	42,246	58,218
Less: Unearned premium reserves	(8,700)	(12,238)
Plus: Loss adjustment expenses reserves	1,116	665
Claim liability reported on Balance Sheet, before reinsurance	34,662	46,645
Reinsurance recoverable reported on Balance Sheet	$ 8,414	$ 9,424
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	19	22
Remaining weighted-average contract period (in years)	23 years	24 years
Gross insured contractual payments outstanding:
Principal	$ 1,925,546	$ 1,998,861
Interest	7,175,101	7,182,715
Total	9,100,647	9,181,576
Gross undiscounted claim liability	1,399,682	1,412,976
Discount, gross claim liability	660,579	643,897
Gross claim liability before all subrogation and before reinsurance	739,103	769,079
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(45,560)	(53,070)
Discount, other subrogation	7,328	8,349
Discounted other subrogation, before reinsurance	(38,232)	(44,721)
Gross claim liability, net of all subrogation and discounts, before reinsurance	700,871	724,358
Less: Unearned premium reserves	(43,963)	(46,086)
Plus: Loss adjustment expenses reserves	8,823	13,331
Claim liability reported on Balance Sheet, before reinsurance	665,731	691,603
Reinsurance recoverable reported on Balance Sheet	$ 38,028	$ 38,465
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	149	179
Remaining weighted-average contract period (in years)	13 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 5,902,142	$ 6,862,281
Interest	2,284,742	2,469,765
Total	8,186,884	9,332,046
Gross undiscounted claim liability	2,474,435	6,409,340
Discount, gross claim liability	680,162	616,559
Gross claim liability before all subrogation and before reinsurance	1,794,273	5,792,781
Less:
Gross RMBS subrogation	(1,851,859)	(1,857,502)
Discount, RMBS subrogation	35,474	23,115
Discounted RMBS subrogation, before reinsurance	(1,816,385)	(1,834,387)
Less:
Gross other subrogation	(645,275)	(743,456)
Discount, other subrogation	64,553	67,045
Discounted other subrogation, before reinsurance	(580,722)	(676,411)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(602,834)	3,281,983
Less: Unearned premium reserves	(57,120)	(64,786)
Plus: Loss adjustment expenses reserves	60,217	56,037
Claim liability reported on Balance Sheet, before reinsurance	(599,737)	3,273,234
Reinsurance recoverable reported on Balance Sheet	$ (12,012)	$ (11,988)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	4	4
Remaining weighted-average contract period (in years)	4 years	4 years
Gross insured contractual payments outstanding:
Principal	$ 43,972	$ 48,562
Interest	15,231	16,332
Total	59,203	64,894
Gross undiscounted claim liability	59,171	64,863
Discount, gross claim liability	5,095	4,739
Gross claim liability before all subrogation and before reinsurance	54,076	60,124
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(13,095)	(13,191)
Discount, other subrogation	4,037	3,709
Discounted other subrogation, before reinsurance	(9,058)	(9,482)
Gross claim liability, net of all subrogation and discounts, before reinsurance	45,018	50,642
Less: Unearned premium reserves	(242)	(276)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	44,776	50,366
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0