Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - Successor [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	$ 1,988,778	$ 1,491,493
Less than 12 Months, Gross Unrealized Loss	86,480	29,944
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	633,662	788,764
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	21,563	49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,622,440	2,280,257
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	108,043	79,642
Short-Term [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	251,926	65,176
Less than 12 Months, Gross Unrealized Loss	209	44
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	251,926	65,176
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	209	44
Fixed Income Investments And Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	108,043	79,642
Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	1,736,852	1,426,317
Less than 12 Months, Gross Unrealized Loss	86,271	29,900
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	633,662	788,764
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	21,563	49,698
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	2,370,514	2,215,081
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	107,834	79,598
Fixed Income Securities [Member] | Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	667,335	98,147
Less than 12 Months, Gross Unrealized Loss	68,578	2,045
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	32,525	122,928
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	822	5,160
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	699,860	221,075
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	69,400	7,205
Fixed Income Securities [Member] | Corporate Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	292,028	963,513
Less than 12 Months, Gross Unrealized Loss	3,377	20,232
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	87,272	6,492
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	2,094	328
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	379,300	970,005
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	5,471	20,560
Fixed Income Securities [Member] | Foreign Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	8,122	5,063
Less than 12 Months, Gross Unrealized Loss	81	100
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	1,700	0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	30	0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	9,822	5,063
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	111	100
Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	74,188	6,037
Less than 12 Months, Gross Unrealized Loss	1,653	93
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	5,525	5,045
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	100	4
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	79,713	11,082
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,753	97
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
Less than 12 Months, Gross Unrealized Loss		3
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		4,060
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		3
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	668,524	226,889
Less than 12 Months, Gross Unrealized Loss	12,524	7,201
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	418,617	550,807
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	17,958	36,584
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,087,141	777,696
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	30,482	43,785
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value		6,986
Less than 12 Months, Gross Unrealized Loss		23
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		25,780
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments		197
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		32,766
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	220
Fixed Income Securities [Member] | Other Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 Months, Fair Value	26,655	115,622
Less than 12 Months, Gross Unrealized Loss	58	203
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	88,023	77,712
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	559	7,425
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	114,678	193,334
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	617	7,628
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Securities Pledged as Collateral [Member] | Fixed Income Securities [Member] | U.S. Government Obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 0	$ 0