Investments - Summary of Gross Unrealized Losses and Fair Values of Ambac's Available-for-Sale Investments (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 816	$ 401
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25	88
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	841	489
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	187	201
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	187	201
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	629	200
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	7	4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	25	88
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	654	288
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	8	5
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	25	13
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	6	10
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	31	23
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	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	17	36
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1	2
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	0	2
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	17	38
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1	2
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	543	63
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	6	2
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	543	68
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6	2
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	3	20
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	3	20
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	14	5
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	14	5
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
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, 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	0	7
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	27	53
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	15	63
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	42	116
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	1
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	0	2
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	4	7
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	4	10
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
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
Fixed Income Securities [Member] | Commercial 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	7
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments		0
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