Fair Value Measurements - Summary of Carrying Amount and Fair Value of Ambac's Financial Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Financial assets:
Debt Securities, Available-for-sale	$ 1,710	$ 1,395
Other Investments	475	568
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	274	61	$ 23	$ 35
Derivative assets	26	28
Financial liabilities:
Derivative liabilities	$ 35	$ 38
Estimated Future Premium Payments Weighted Average Discounted Rate	3.20%	3.00%
Interest Rate Swap [Member]
Financial assets:
Derivative assets	$ 25	$ 27
Financial liabilities:
Derivative liabilities	35	38
Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,167	1,967
Loans, at fair value	1,663	1,829
Derivative assets	226	239
Financial liabilities:
Derivative liabilities	1,197	1,048
Long-term Debt	$ 2,710	$ 2,788
Estimated Future Premium Payments Weighted Average Discounted Rate	6.30%	6.80%
Variable Interest Entity [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	$ 190	$ 190
Financial liabilities:
Derivative liabilities	1,197	1,048
Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	426	507
Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	303	224
Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	139	137
Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	250	238
US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	82	65
Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	100	76
Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	745	598
Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,072	1,828
Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	72	43
Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	95	96
Debt Securities, Trading	0	43
Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	19	0
Warrant [Member]
Financial assets:
Derivative assets	1	1
Carrying Amount [Member]
Financial assets:
Other Investments	77	61
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	28	44
Loans	2	10
Total financial assets	7,022	6,726
Financial liabilities:
Liabilities for net financial guarantees written	292	159
Long-term Debt	983	1,065
Total financial liabilities	5,474	5,418
Carrying Amount [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	246	17
Loans, at fair value	1,663	1,829
Financial liabilities:
Long-term Debt	2,967	3,107
Carrying Amount [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	475	568
Carrying Amount [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	426	507
Carrying Amount [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,072	1,828
Carrying Amount [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	95	96
Debt Securities, Trading	0	43
Carrying Amount [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	35	38
Carrying Amount [Member] | Interest Rate Swap [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	190
Financial liabilities:
Derivative liabilities	1,197	1,048
Carrying Amount [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	36	49
Carrying Amount [Member] | Warrant [Member]
Financial assets:
Derivative assets	1	1
Total Fair Value [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	28	44
Other Assets [Member] | Carrying Amount [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	25	27
Fair Value Measured at Net Asset Value Per Share [Member]
Financial assets:
Other Investments	386	494
Level 1 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	27	43
Total financial assets	979	833
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	0	0
Total financial liabilities	0	0
Level 1 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	246	17
Loans	0	0
Financial liabilities:
Long-term Debt	0	0
Level 1 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	77	61
Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	421	506
Level 1 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 1 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Debt Securities, Trading	0	0
Level 1 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Interest Rate Swap [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	190
Financial liabilities:
Derivative liabilities	0	0
Level 1 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 1 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 1 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 2 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	2	1
Total financial assets	1,795	1,615
Financial liabilities:
Liabilities for net financial guarantees written	0	0
Long-term Debt	679	864
Total financial liabilities	4,671	4,942
Level 2 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash		0
Loans	0	0
Financial liabilities:
Long-term Debt	2,760	2,992
Level 2 [Member] | Variable Interest Entity [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets		190
Level 2 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	5	1
Level 2 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Level 2 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	95	96
Debt Securities, Trading	0	43
Level 2 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	35	38
Level 2 [Member] | Interest Rate Swap [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	190	0
Financial liabilities:
Derivative liabilities	1,197	1,048
Level 2 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	36	49
Level 2 [Member] | Loans Receivable [Member]
Financial assets:
Loans	0	0
Level 2 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	1
Level 3 [Member]
Financial assets:
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Total financial assets	3,850	3,772
Financial liabilities:
Liabilities for net financial guarantees written	788	476
Long-term Debt	18	14
Total financial liabilities	1,026	644
Level 3 [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	0	0
Loans	1,663	1,829
Financial liabilities:
Long-term Debt	220	154
Level 3 [Member] | Variable Interest Entity [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	0	0
Level 3 [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	0	0
Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	0	0
Level 3 [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,072	1,828
Level 3 [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Debt Securities, Trading	0	0
Level 3 [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Interest Rate Swap [Member] | Variable Interest Entity [Member]
Financial liabilities:
Derivative liabilities	0	0
Level 3 [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	0	0
Level 3 [Member] | Loans Receivable [Member]
Financial assets:
Loans	2	10
Level 3 [Member] | Warrant [Member]
Financial assets:
Derivative assets	1	1
Level 3 [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	25	26
Municipal Bonds [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	99	102
Corporate Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	745	598
Debt Security, Government, Non-US [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	100	76
US Government Debt Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	82	65
Residential Mortgage-Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	250	238
Collateralized Debt Obligations [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	139	137
Asset-backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	303	224
Commercial Mortgage Backed Securities [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	19	15
Fair Value, Recurring [Member]
Financial assets:
Total financial assets	7,010	6,715
Financial liabilities:
Liabilities for net financial guarantees written	788	476
Long-term Debt	697	878
Total financial liabilities	5,697	5,586
Fair Value, Recurring [Member] | Variable Interest Entity [Member]
Financial assets:
Restricted cash	246	17
Loans	1,663	1,829
Financial liabilities:
Long-term Debt	2,980	3,145
Fair Value, Recurring [Member] | Other Debt Obligations [Member]
Financial assets:
Other Investments	463	556
Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Short term investments	426	507
Fair Value, Recurring [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	303	224
Fair Value, Recurring [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	139	137
Fair Value, Recurring [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	250	238
Fair Value, Recurring [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	82	65
Fair Value, Recurring [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	100	76
Fair Value, Recurring [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	745	598
Fair Value, Recurring [Member] | Corporate Debt Securities [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	2,072	1,828
Fair Value, Recurring [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	99	102
Fair Value, Recurring [Member] | Municipal Bonds [Member] | Variable Interest Entity [Member]
Financial assets:
Debt Securities, Available-for-sale	95	96
Debt Securities, Trading	0	43
Fair Value, Recurring [Member] | Interest Rate Swap [Member]
Financial liabilities:
Derivative liabilities	35	38
Fair Value, Recurring [Member] | Interest Rate Swap [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	190	190
Financial liabilities:
Derivative liabilities	1,197	1,048
Fair Value, Recurring [Member] | Currency Swaps [Member] | Variable Interest Entity [Member]
Financial assets:
Derivative assets	36	49
Fair Value, Recurring [Member] | Loans Receivable [Member]
Financial assets:
Loans	2	10
Fair Value, Recurring [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	19
Fair Value, Recurring [Member] | Other Assets [Member] | Interest Rate Swap [Member]
Financial assets:
Derivative assets	25	27
Fair Value, Recurring [Member] | Level 1 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	82	65
Fair Value, Recurring [Member] | Level 1 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	100	76
Fair Value, Recurring [Member] | Level 1 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	235	157
Fair Value, Recurring [Member] | Level 2 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	139	137
Fair Value, Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	250	238
Fair Value, Recurring [Member] | Level 2 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 2 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	726	585
Fair Value, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	99	102
Fair Value, Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	19	15
Fair Value, Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	68	67
Fair Value, Recurring [Member] | Level 3 [Member] | Collateralized Debt Obligations [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | US Government Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Debt Security, Government, Non-US [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Corporate Debt Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	19	12
Fair Value, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Fair Value, Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member]
Financial assets:
Debt Securities, Available-for-sale	27	64
Collateral Pledged [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	27	64
Collateral Pledged [Member] | Short-term Investments [Member] | Carrying Amount [Member]
Financial assets:
Debt Securities, Available-for-sale	27	64
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	27	64
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 1 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	27	64
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 2 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	0	0
Collateral Pledged [Member] | Fair Value, Recurring [Member] | Level 3 [Member] | Short-term Investments [Member]
Financial assets:
Debt Securities, Available-for-sale	$ 0	$ 0