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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 313,685 $ 1,139,101
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 981 27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 26,158 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 368 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position 339,843 1,499,044
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1,349 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 9,093  
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses 91  
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 137,514 115,374
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 23 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 137,514 115,374
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 23 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 1,349  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 176,171 1,023,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 958 27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 26,158 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 368 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position 202,329 1,383,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1,326 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 0 537,904
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 10,436 28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 280 1,198
Debt Securities, Available-for-sale, Unrealized Loss Position 10,436 566,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 280 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 63,985 306,506
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 267 8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 4,987 190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 21 8,570
Debt Securities, Available-for-sale, Unrealized Loss Position 68,972 496,779
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 288 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 1,122 1,161
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 312 5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1 60
Debt Securities, Available-for-sale, Unrealized Loss Position 1,434 6,324
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 2 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 1,741 5,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 61 135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 2,273 58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 13 478
Debt Securities, Available-for-sale, Unrealized Loss Position 4,014 64,138
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 74 613
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 4,112 34,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 58 793
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 4,112 34,852
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 58 793
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 99,144 123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 542 1,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 2,969 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 31 0
Debt Securities, Available-for-sale, Unrealized Loss Position 102,113 123,848
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 573 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 6,067 13,813
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 29 33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 5,181 77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 22 591
Debt Securities, Available-for-sale, Unrealized Loss Position 11,248 91,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 51 $ 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