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	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) Policies	Dec. 31, 2016 USD ($) Policies	Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Loss Reserves Ceded To Reinsurers	$ 46,023	$ 30,767	$ 24,298	$ 44,059
Ceded Loss And Loss Expenses Paid Not Yet Recovered	$ (47)	$ (349)	$ 143
Number of policies | Policies	292	282
Remaining weighted-average contract period (in years)	17 years	16 years
Gross insured contractual payments outstanding:
Principal	$ 12,543,420	$ 13,400,162
Interest	11,140,001	11,026,059
Total	23,683,421	24,426,221
Gross undiscounted claim liability	7,960,610	7,636,110
Discount, gross claim liability	1,250,990	1,305,366
Gross claim liability before all subrogation and before reinsurance	6,709,620	6,330,744
Less:
Gross RMBS subrogation	(1,861,859)	(1,926,165)
Discount, RMBS subrogation	17,743	19,130
Discounted RMBS subrogation, before reinsurance	(1,844,116)	(1,907,035)
Less:
Gross other subrogation	(903,883)	(739,471)
Discount, other subrogation	81,521	80,079
Discounted other subrogation, before reinsurance	(822,362)	(659,392)
Gross claim liability, net of all subrogation and discounts, before reinsurance	4,043,142	3,764,317
Less: Unearned premium reserves	(141,307)	(143,141)
Plus: Loss adjustment expenses reserves	98,520	74,862
Claim liability reported on Balance Sheet, before reinsurance	4,000,355	3,696,038
Reinsurance recoverable reported on Balance Sheet	$ 45,976	$ 30,418
I/SL [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	21	19
Remaining weighted-average contract period (in years)	9 years	9 years
Gross insured contractual payments outstanding:
Principal	$ 949,992	$ 918,456
Interest	258,114	345,802
Total	1,208,106	1,264,258
Gross undiscounted claim liability	2,341	3,439
Discount, gross claim liability	215	314
Gross claim liability before all subrogation and before reinsurance	2,126	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	2,126	3,125
Less: Unearned premium reserves	(1,243)	(2,394)
Plus: Loss adjustment expenses reserves	15,716	6,621
Claim liability reported on Balance Sheet, before reinsurance	16,599	7,352
Reinsurance recoverable reported on Balance Sheet	$ 134	$ 120
IA [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	24	22
Remaining weighted-average contract period (in years)	23 years	8 years
Gross insured contractual payments outstanding:
Principal	$ 573,865	$ 733,036
Interest	602,701	199,631
Total	1,176,566	932,667
Gross undiscounted claim liability	52,976	21,175
Discount, gross claim liability	10,757	1,243
Gross claim liability before all subrogation and before reinsurance	42,219	19,932
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(10,276)	0
Discount, other subrogation	6,836	0
Discounted other subrogation, before reinsurance	(3,440)	0
Gross claim liability, net of all subrogation and discounts, before reinsurance	38,779	19,932
Less: Unearned premium reserves	(10,205)	(1,807)
Plus: Loss adjustment expenses reserves	3,499	339
Claim liability reported on Balance Sheet, before reinsurance	32,073	18,464
Reinsurance recoverable reported on Balance Sheet	$ 3,796	$ 6,063
II [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	31	26
Remaining weighted-average contract period (in years)	9 years	30 years
Gross insured contractual payments outstanding:
Principal	$ 1,483,272	$ 1,992,543
Interest	482,551	7,080,969
Total	1,965,823	9,073,512
Gross undiscounted claim liability	94,609	547,550
Discount, gross claim liability	12,371	331,234
Gross claim liability before all subrogation and before reinsurance	82,238	216,316
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(10,974)	(14,529)
Discount, other subrogation	2,996	6,526
Discounted other subrogation, before reinsurance	(7,978)	(8,003)
Gross claim liability, net of all subrogation and discounts, before reinsurance	74,260	208,313
Less: Unearned premium reserves	(14,057)	(49,578)
Plus: Loss adjustment expenses reserves	265	777
Claim liability reported on Balance Sheet, before reinsurance	60,468	159,512
Reinsurance recoverable reported on Balance Sheet	$ 15,086	$ 2,737
III [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	23	43
Remaining weighted-average contract period (in years)	25 years	17 years
Gross insured contractual payments outstanding:
Principal	$ 1,913,092	$ 1,779,889
Interest	7,156,992	1,110,051
Total	9,070,084	2,889,940
Gross undiscounted claim liability	1,191,399	861,455
Discount, gross claim liability	528,293	256,108
Gross claim liability before all subrogation and before reinsurance	663,106	605,347
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(57,855)	(118,272)
Discount, other subrogation	8,906	13,426
Discounted other subrogation, before reinsurance	(48,949)	(104,846)
Gross claim liability, net of all subrogation and discounts, before reinsurance	614,157	500,501
Less: Unearned premium reserves	(44,894)	(31,785)
Plus: Loss adjustment expenses reserves	5,680	11,036
Claim liability reported on Balance Sheet, before reinsurance	574,943	479,752
Reinsurance recoverable reported on Balance Sheet	$ 39,250	$ 39,352
IV [Member]
Schedule of Insured Financial Obligations with Credit Deterioration [Line Items]
Number of policies | Policies	189	169
Remaining weighted-average contract period (in years)	14 years	14 years
Gross insured contractual payments outstanding:
Principal	$ 7,573,829	$ 7,926,991
Interest	2,621,448	2,275,421
Total	10,195,277	10,202,412
Gross undiscounted claim liability	6,551,752	6,139,060
Discount, gross claim liability	694,892	710,608
Gross claim liability before all subrogation and before reinsurance	5,856,860	5,428,452
Less:
Gross RMBS subrogation	(1,861,859)	(1,926,165)
Discount, RMBS subrogation	17,743	19,130
Discounted RMBS subrogation, before reinsurance	(1,844,116)	(1,907,035)
Less:
Gross other subrogation	(811,521)	(593,919)
Discount, other subrogation	59,072	56,273
Discounted other subrogation, before reinsurance	(752,449)	(537,646)
Gross claim liability, net of all subrogation and discounts, before reinsurance	3,260,295	2,983,771
Less: Unearned premium reserves	(70,613)	(57,194)
Plus: Loss adjustment expenses reserves	73,360	56,089
Claim liability reported on Balance Sheet, before reinsurance	3,263,042	2,982,666
Reinsurance recoverable reported on Balance Sheet	$ (12,290)	$ (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	$ 49,370	$ 49,247
Interest	18,195	14,185
Total	67,565	63,432
Gross undiscounted claim liability	67,533	63,431
Discount, gross claim liability	4,462	5,859
Gross claim liability before all subrogation and before reinsurance	63,071	57,572
Less:
Gross RMBS subrogation	0	0
Discount, RMBS subrogation	0	0
Discounted RMBS subrogation, before reinsurance	0	0
Less:
Gross other subrogation	(13,257)	(12,751)
Discount, other subrogation	3,711	3,854
Discounted other subrogation, before reinsurance	(9,546)	(8,897)
Gross claim liability, net of all subrogation and discounts, before reinsurance	53,525	48,675
Less: Unearned premium reserves	(295)	(383)
Plus: Loss adjustment expenses reserves	0	0
Claim liability reported on Balance Sheet, before reinsurance	53,230	48,292
Reinsurance recoverable reported on Balance Sheet	$ 0	$ 0