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	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Policies	Dec. 31, 2017 USD ($) Policies	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 38,735	$ 40,658	$ 34,691	$ 30,767
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 90	$ 339	$ (418)
Number of policies | Policies	250	277
Remaining weighted-average contract period (in years)	16 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 10,599,933	$ 11,687,070
Interest	10,917,345	11,197,612
Total	21,517,278	22,884,682
Gross undiscounted claim liability	4,179,258	8,025,561
Discount, gross claim liability	1,398,025	1,291,005
Gross claim liability before all subrogation and before reinsurance	2,781,233	6,734,556
Less:
Gross RMBS subrogation	(1,861,894)	(1,857,502)
Discount, RMBS subrogation	28,384	23,115
Discounted RMBS subrogation, before reinsurance	(1,833,510)	(1,834,387)
Less:
Gross other subrogation	(750,538)	(827,078)
Discount, other subrogation	88,909	86,822
Discounted other subrogation, before reinsurance	(661,629)	(740,256)
Gross claim liability, net of all subrogation and discounts, before reinsurance	286,094	4,159,913
Less: Unearned premium reserves	(128,741)	(135,502)
Plus: Loss adjustment expenses reserves	86,970	89,391
Claim liability reported on Balance Sheet, before reinsurance	244,323	4,113,802
Reinsurance recoverable reported on Balance Sheet	$ 38,825	$ 40,997
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	25	26
Remaining weighted-average contract period (in years)	10 years	10 years
Gross insured contractual payments outstanding:
Principal	$ 967,427	$ 1,046,267
Interest	500,680	531,657
Total	1,468,107	1,577,924
Gross undiscounted claim liability	4,264	4,434
Discount, gross claim liability	514	465
Gross claim liability before all subrogation and before reinsurance	3,750	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	3,750	3,969
Less: Unearned premium reserves	(1,586)	(2,126)
Plus: Loss adjustment expenses reserves	16,141	16,116
Claim liability reported on Balance Sheet, before reinsurance	18,305	17,959
Reinsurance recoverable reported on Balance Sheet	$ 238	$ 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	$ 576,858	$ 531,190
Interest	601,901	584,098
Total	1,178,759	1,115,288
Gross undiscounted claim liability	56,682	56,659
Discount, gross claim liability	13,866	13,095
Gross claim liability before all subrogation and before reinsurance	42,816	43,564
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(8,604)	(7,990)
Discount, other subrogation	5,810	5,169
Discounted other subrogation, before reinsurance	(2,794)	(2,821)
Gross claim liability, net of all subrogation and discounts, before reinsurance	40,022	40,743
Less: Unearned premium reserves	(10,092)	(9,990)
Plus: Loss adjustment expenses reserves	3,082	3,242
Claim liability reported on Balance Sheet, before reinsurance	33,012	33,995
Reinsurance recoverable reported on Balance Sheet	$ 4,448	$ 4,894
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	15	26
Remaining weighted-average contract period (in years)	10 years	10 years
Gross insured contractual payments outstanding:
Principal	$ 642,281	$ 1,199,909
Interest	243,259	413,045
Total	885,540	1,612,954
Gross undiscounted claim liability	59,584	77,289
Discount, gross claim liability	9,724	12,250
Gross claim liability before all subrogation and before reinsurance	49,860	65,039
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(4)	(9,371)
Discount, other subrogation	0	2,550
Discounted other subrogation, before reinsurance	(4)	(6,821)
Gross claim liability, net of all subrogation and discounts, before reinsurance	49,856	58,218
Less: Unearned premium reserves	(9,810)	(12,238)
Plus: Loss adjustment expenses reserves	582	665
Claim liability reported on Balance Sheet, before reinsurance	40,628	46,645
Reinsurance recoverable reported on Balance Sheet	$ 8,368	$ 9,424
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	23	22
Remaining weighted-average contract period (in years)	23 years	24 years
Gross insured contractual payments outstanding:
Principal	$ 2,013,845	$ 1,998,861
Interest	7,195,260	7,182,715
Total	9,209,105	9,181,576
Gross undiscounted claim liability	1,449,879	1,412,976
Discount, gross claim liability	693,598	643,897
Gross claim liability before all subrogation and before reinsurance	756,281	769,079
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(53,703)	(53,070)
Discount, other subrogation	9,497	8,349
Discounted other subrogation, before reinsurance	(44,206)	(44,721)
Gross claim liability, net of all subrogation and discounts, before reinsurance	712,075	724,358
Less: Unearned premium reserves	(44,754)	(46,086)
Plus: Loss adjustment expenses reserves	11,146	13,331
Claim liability reported on Balance Sheet, before reinsurance	678,467	691,603
Reinsurance recoverable reported on Balance Sheet	$ 38,603	$ 38,465
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	157	179
Remaining weighted-average contract period (in years)	12 years	13 years
Gross insured contractual payments outstanding:
Principal	$ 6,350,960	$ 6,862,281
Interest	2,359,913	2,469,765
Total	8,710,873	9,332,046
Gross undiscounted claim liability	2,543,988	6,409,340
Discount, gross claim liability	675,210	616,559
Gross claim liability before all subrogation and before reinsurance	1,868,778	5,792,781
Less:
Gross RMBS subrogation	(1,861,894)	(1,857,502)
Discount, RMBS subrogation	28,384	23,115
Discounted RMBS subrogation, before reinsurance	(1,833,510)	(1,834,387)
Less:
Gross other subrogation	(675,089)	(743,456)
Discount, other subrogation	69,630	67,045
Discounted other subrogation, before reinsurance	(605,459)	(676,411)
Gross claim liability, net of all subrogation and discounts, before reinsurance	(570,191)	3,281,983
Less: Unearned premium reserves	(62,241)	(64,786)
Plus: Loss adjustment expenses reserves	56,019	56,037
Claim liability reported on Balance Sheet, before reinsurance	(576,413)	3,273,234
Reinsurance recoverable reported on Balance Sheet	$ (12,832)	$ (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	$ 48,562	$ 48,562
Interest	16,332	16,332
Total	64,894	64,894
Gross undiscounted claim liability	64,861	64,863
Discount, gross claim liability	5,113	4,739
Gross claim liability before all subrogation and before reinsurance	59,748	60,124
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(13,138)	(13,191)
Discount, other subrogation	3,972	3,709
Discounted other subrogation, before reinsurance	(9,166)	(9,482)
Gross claim liability, net of all subrogation and discounts, before reinsurance	50,582	50,642
Less: Unearned premium reserves	(258)	(276)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	50,324	50,366
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0