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	12 Months Ended
	Dec. 31, 2017 USD ($) Policies	Dec. 31, 2016 USD ($) Policies	Dec. 31, 2015 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	277	282
Remaining weighted-average contract period (in years)	17 years	16 years
Gross insured contractual payments outstanding:
Principal	$ 11,687,070	$ 13,400,162
Interest	11,197,612	11,026,059
Total	22,884,682	24,426,221
Gross undiscounted claim liability	8,025,561	7,636,110
Discount, gross claim liability	1,291,005	1,305,366
Gross claim liability before all subrogation and before reinsurance	6,734,556	6,330,744
Less:
Gross RMBS subrogation	(1,857,502)	(1,926,165)
Discount, RMBS subrogation	23,115	19,130
Discounted RMBS subrogation, before reinsurance	(1,834,387)	(1,907,035)
Less:
Gross other subrogation	(827,078)	(739,471)
Discount, other subrogation	86,822	80,079
Discounted other subrogation, before reinsurance	(740,256)	(659,392)
Gross claim liability, net of all subrogation and discounts, before reinsurance	4,159,913	3,764,317
Less: Unearned premium reserves	(135,502)	(143,141)
Plus: Loss adjustment expenses reserves	89,391	74,862
Claim liability reported on Balance Sheet, before reinsurance	4,113,802	3,696,038
Reinsurance recoverable reported on Balance Sheet	40,997	30,418
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ 339	$ (349)	$ (60)
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	26	19
Remaining weighted-average contract period (in years)	10 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 1,046,267	$ 918,456
Interest	531,657	345,802
Total	1,577,924	1,264,258
Gross undiscounted claim liability	4,434	3,439
Discount, gross claim liability	465	314
Gross claim liability before all subrogation and before reinsurance	3,969	3,125
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	3,969	3,125
Less: Unearned premium reserves	(2,126)	(2,394)
Plus: Loss adjustment expenses reserves	16,116	6,621
Claim liability reported on Balance Sheet, before reinsurance	17,959	7,352
Reinsurance recoverable reported on Balance Sheet	$ 202	$ 120
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	20	22
Remaining weighted-average contract period (in years)	23 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 531,190	$ 733,036
Interest	584,098	199,631
Total	1,115,288	932,667
Gross undiscounted claim liability	56,659	21,175
Discount, gross claim liability	13,095	1,243
Gross claim liability before all subrogation and before reinsurance	43,564	19,932
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(7,990)	0
Discount, other subrogation	5,169	0
Discounted other subrogation, before reinsurance	(2,821)	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	40,743	19,932
Less: Unearned premium reserves	(9,990)	(1,807)
Plus: Loss adjustment expenses reserves	3,242	339
Claim liability reported on Balance Sheet, before reinsurance	33,995	18,464
Reinsurance recoverable reported on Balance Sheet	$ 4,894	$ 6,063
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	26	26
Remaining weighted-average contract period (in years)	10 years	30 years
Gross insured contractual payments outstanding:
Principal	$ 1,199,909	$ 1,992,543
Interest	413,045	7,080,969
Total	1,612,954	9,073,512
Gross undiscounted claim liability	77,289	547,550
Discount, gross claim liability	12,250	331,234
Gross claim liability before all subrogation and before reinsurance	65,039	216,316
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(9,371)	(14,529)
Discount, other subrogation	2,550	6,526
Discounted other subrogation, before reinsurance	(6,821)	(8,003)
Gross claim liability, net of all subrogation and discounts, before reinsurance	58,218	208,313
Less: Unearned premium reserves	(12,238)	(49,578)
Plus: Loss adjustment expenses reserves	665	777
Claim liability reported on Balance Sheet, before reinsurance	46,645	159,512
Reinsurance recoverable reported on Balance Sheet	$ 9,424	$ 2,737
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	22	43
Remaining weighted-average contract period (in years)	24 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,998,861	$ 1,779,889
Interest	7,182,715	1,110,051
Total	9,181,576	2,889,940
Gross undiscounted claim liability	1,412,976	861,455
Discount, gross claim liability	643,897	256,108
Gross claim liability before all subrogation and before reinsurance	769,079	605,347
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(53,070)	(118,272)
Discount, other subrogation	8,349	13,426
Discounted other subrogation, before reinsurance	(44,721)	(104,846)
Gross claim liability, net of all subrogation and discounts, before reinsurance	724,358	500,501
Less: Unearned premium reserves	(46,086)	(31,785)
Plus: Loss adjustment expenses reserves	13,331	11,036
Claim liability reported on Balance Sheet, before reinsurance	691,603	479,752
Reinsurance recoverable reported on Balance Sheet	$ 38,465	$ 39,352
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	179	169
Remaining weighted-average contract period (in years)	13 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 6,862,281	$ 7,926,991
Interest	2,469,765	2,275,421
Total	9,332,046	10,202,412
Gross undiscounted claim liability	6,409,340	6,139,060
Discount, gross claim liability	616,559	710,608
Gross claim liability before all subrogation and before reinsurance	5,792,781	5,428,452
Less:
Gross RMBS subrogation	(1,857,502)	(1,926,165)
Discount, RMBS subrogation	23,115	19,130
Discounted RMBS subrogation, before reinsurance	(1,834,387)	(1,907,035)
Less:
Gross other subrogation	(743,456)	(593,919)
Discount, other subrogation	67,045	56,273
Discounted other subrogation, before reinsurance	(676,411)	(537,646)
Gross claim liability, net of all subrogation and discounts, before reinsurance	3,281,983	2,983,771
Less: Unearned premium reserves	(64,786)	(57,194)
Plus: Loss adjustment expenses reserves	56,037	56,089
Claim liability reported on Balance Sheet, before reinsurance	3,273,234	2,982,666
Reinsurance recoverable reported on Balance Sheet	$ (11,988)	$ (17,854)
V [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	4	3
Remaining weighted-average contract period (in years)	4 years	5 years
Gross insured contractual payments outstanding:
Principal	$ 48,562	$ 49,247
Interest	16,332	14,185
Total	64,894	63,432
Gross undiscounted claim liability	64,863	63,431
Discount, gross claim liability	4,739	5,859
Gross claim liability before all subrogation and before reinsurance	60,124	57,572
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(13,191)	(12,751)
Discount, other subrogation	3,709	3,854
Discounted other subrogation, before reinsurance	(9,482)	(8,897)
Gross claim liability, net of all subrogation and discounts, before reinsurance	50,642	48,675
Less: Unearned premium reserves	(276)	(383)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	50,366	48,292
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0