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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($)
$ in Thousands
Jun. 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 $ 144,371 $ 1,139,101
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 942 27,298
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 70,611 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1,017 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position 214,982 1,499,044
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1,959 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 45,718  
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses 573  
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 4,695 115,374
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 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 4,695 115,374
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 2 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 1,959  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 139,676 1,023,727
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 940 27,201
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 70,611 359,943
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 1,017 10,897
Debt Securities, Available-for-sale, Unrealized Loss Position 210,287 1,383,670
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 1,957 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 21,593 537,904
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 325 15,878
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 25,981 28,533
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 584 1,198
Debt Securities, Available-for-sale, Unrealized Loss Position 47,574 566,437
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 909 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 22,520 306,506
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 78 8,634
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 34,130 190,273
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 359 8,570
Debt Securities, Available-for-sale, Unrealized Loss Position 56,650 496,779
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 437 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 281 1,161
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 1
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3,061 5,163
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 14 60
Debt Securities, Available-for-sale, Unrealized Loss Position 3,342 6,324
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 14 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 2,833 5,643
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 21 135
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 2,272 58,495
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 18 478
Debt Securities, Available-for-sale, Unrealized Loss Position 5,105 64,138
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 39 613
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 4,500 34,852
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 62 793
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 4,500 34,852
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 62 793
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 83,555 123,848
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 441 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 83,555 123,848
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 441 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 4,394 13,813
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 13 33
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 5,167 77,479
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 42 591
Debt Securities, Available-for-sale, Unrealized Loss Position 9,561 91,292
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax 55 $ 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