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Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 768 $ 816
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 14 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 41 25
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 1 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 810 841
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 16 8
Debt Securities, Available for sale, Unrealized Loss Position, Including Pledged Securities 825  
Debt Securities , Available for sale, Continuous Unrealized Loss Position, 783  
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 114 187
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 13 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 128 187
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 654 629
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 14 7
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 28 25
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 1 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 682 654
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 16 8
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 117 25
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 3 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 2 6
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 118 31
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 3 0
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 25 17
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 2 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 27 17
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 1 1
Fixed Income Securities [Member] | US Government Debt Securities [Member] | Asset Pledged as Collateral [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 15  
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0  
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 15  
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0  
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 363 543
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 8 6
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 17 0
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 1 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 380 543
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 9 6
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 75 3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3 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 78 3
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 2 0
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 0 14
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 1 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 2 14
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
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 68 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 3 15
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 71 42
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 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 6 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 0 4
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 6 4
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments $ 0 $ 0