Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,491,493	$ 2,281,230
Less than 12 Months, Gross Unrealized Loss	29,944	82,799
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	788,764	463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	49,698	21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,280,257	2,744,550
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,642	104,206
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	65,176	9,982
Less than 12 Months, Gross Unrealized Loss	44	1
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	65,176	9,982
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	44	1
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,491,493	2,216,675
Less than 12 Months, Gross Unrealized Loss	29,944	82,742
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	788,764	463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	49,698	21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,280,257	2,679,995
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,642	104,149
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,426,317	2,206,693
Less than 12 Months, Gross Unrealized Loss	29,900	82,741
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	788,764	463,320
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	49,698	21,407
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,215,081	2,670,013
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	79,598	104,148
Fixed Income Securities [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	98,147	117,008
Less than 12 Months, Gross Unrealized Loss	2,045	2,070
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	122,928	114,708
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	5,160	6,118
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	221,075	231,716
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,205	8,188
Fixed Income Securities [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	963,513	938,916
Less than 12 Months, Gross Unrealized Loss	20,232	21,331
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	6,492	92,581
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	328	3,001
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	970,005	1,031,497
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	20,560	24,332
Fixed Income Securities [Member] | Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	5,063	34,904
Less than 12 Months, Gross Unrealized Loss	100	1,018
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	8,584
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	805
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	5,063	43,488
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	100	1,823
Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	6,037	2,938
Less than 12 Months, Gross Unrealized Loss	93	18
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,045	10,658
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	4	170
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	11,082	13,596
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	97	188
Fixed Income Securities [Member] | U.S. Agency Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	4,060	0
Less than 12 Months, Gross Unrealized Loss	3	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	4,212
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	27
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	4,060	4,212
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	3	27
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	226,889	584,699
Less than 12 Months, Gross Unrealized Loss	7,201	53,367
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	550,807	213,303
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	36,584	11,250
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	777,696	798,002
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	43,785	64,617
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	6,986	77,538
Less than 12 Months, Gross Unrealized Loss	23	1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	25,780	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	197	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	32,766	77,538
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	220	1,481
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	115,622	450,690
Less than 12 Months, Gross Unrealized Loss	203	3,456
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	77,712	19,274
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	7,425	36
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	193,334	469,964
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	7,628	3,492
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	64,555
Less than 12 Months, Gross Unrealized Loss	0	57
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	64,555
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	57
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	0	64,555
Less than 12 Months, Gross Unrealized Loss	0	57
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	0	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	0	64,555
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 0	$ 57