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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 283,413	$ 1,139,101
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,800	27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Unrealized Loss Position	707,413	1,499,044
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345	38,195
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	121,037
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses	2,086
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	99,429	115,374
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	17	97
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position	99,429	115,374
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	17	97
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,345
Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	183,984	1,023,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	2,783	27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	424,000	359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	5,545	10,897
Debt Securities, Available-for-sale, Unrealized Loss Position	607,984	1,383,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	8,328	38,098
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	20,997	537,904
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	951	15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25,865	28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	836	1,198
Debt Securities, Available-for-sale, Unrealized Loss Position	46,862	566,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,787	17,076
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	39,639	306,506
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	550	8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	256,406	190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,858	8,570
Debt Securities, Available-for-sale, Unrealized Loss Position	296,045	496,779
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4,408	17,204
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	401	1,161
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	4	1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	3,980	5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	22	60
Debt Securities, Available-for-sale, Unrealized Loss Position	4,381	6,324
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	26	61
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	12,793	5,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	360	135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	53,861	58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	204	478
Debt Securities, Available-for-sale, Unrealized Loss Position	66,654	64,138
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	564	613
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	4,375	34,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	122	793
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	243	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1	0
Debt Securities, Available-for-sale, Unrealized Loss Position	4,618	34,852
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	123	793
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	101,939	123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	787	1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	0	0
Debt Securities, Available-for-sale, Unrealized Loss Position	101,939	123,848
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	787	1,727
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	3,840	13,813
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	9	33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	83,645	77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	624	591
Debt Securities, Available-for-sale, Unrealized Loss Position	87,485	91,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	633	$ 624
Collateral Pledged [Member] | Fixed Income Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0
Collateral Pledged [Member] | Fixed Income Securities [Member] | US Government Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 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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