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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 1,139,101	$ 1,988,778
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	27,298	86,480
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	359,943	633,662
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	10,897	21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,499,044	2,622,440
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	38,195	108,043
Below Investment Grade Securities and Non-Rated Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Fair Value	660,063	1,855,694
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses	17,838	100,503
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	115,374	251,926
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	97	209
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	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	115,374	251,926
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	97	209
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		108,043
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,023,727	1,736,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	27,201	86,271
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	359,943	633,662
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	10,897	21,563
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	1,383,670	2,370,514
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	38,098	107,834
Fixed Income Securities [Member] | Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	537,904	667,335
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	15,878	68,578
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	28,533	32,525
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	1,198	822
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	566,437	699,860
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	17,076	69,400
Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	5,643	74,188
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	135	1,653
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	58,495	5,525
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	478	100
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	64,138	79,713
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	613	1,753
Fixed Income Securities [Member] | Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	306,506	292,028
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	8,634	3,377
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	190,273	87,272
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	8,570	2,094
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	496,779	379,300
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	17,204	5,471
Fixed Income Securities [Member] | Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	1,161	8,122
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	81
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	5,163	1,700
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	60	30
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	6,324	9,822
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	61	111
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	34,852	668,524
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	793	12,524
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	418,617
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	0	17,958
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	34,852	1,087,141
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	793	30,482
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	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	123,848
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,727	0
Fixed Income Securities [Member] | Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	13,813	26,655
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	33	58
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	77,479	88,023
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments	591	559
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value	91,292	114,678
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	$ 624	617
Collateral Pledged [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		$ 0

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Aggregate losses of below investment grade securities and non rated securities in a continuous unrealized loss position.

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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