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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, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months $ 401 $ 1,139
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 4 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 88 360
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 1 11
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 489 1,499
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 5 38
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 29 660
Below Investment Grade Securities And Non Rated Securities Continuous Unrealized Loss Position Aggregate Losses 0 18
Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 201 115
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 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 201 115
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 0
Fixed Income Investments And Other Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 5  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 200 1,024
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 4 27
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 88 360
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 1 11
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 288 1,384
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 5 38
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 13 538
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 16
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 10 29
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 0 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 23 566
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 17
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 36 6
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 2 58
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 38 64
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 2 1
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 63 307
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 2 9
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 5 190
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 0 9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 68 497
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 2 17
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 20 1
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 5
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 20 6
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 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 5 35
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 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 5 35
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 1
Fixed Income Securities [Member] | Collateralized Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months 7  
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 7  
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 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 53 124
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer 63 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 116 124
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 1 2
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 2 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 7 77
Available For Sale Securities Continuous Unrealized Loss Position 12 Months Or Longer Aggregate Losses Accumulated In Investments 0 1
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value 10 91
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments 0 $ 1
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] | Short-term Investments [Member]    
Debt Securities, Available-for-sale [Line Items]    
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments $ 0