Variable Interest Entities - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) Entity	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($) Entity	Dec. 31, 2021 USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 2,197	$ 2,197	$ 2,197	$ 2,041	$ 2,041	$ 2,041
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	40	40	40	31	31	31
Available-for-sale Securities	1,710	1,710	1,710	1,395	1,395	1,395
Restricted Cash	12	12	12	14	14	14	$ 5
Assets	8,428	8,428	8,428	7,973	7,973	7,973	12,303
Liabilities	6,997	6,997	6,997	6,647	6,647	6,647
Other Investments	475	475	475	568	568	568
Long-term debt	508	508	508	639	639	639
Derivative assets	26	26	26	27	27	27
Other assets	129	129	129	112	112	112
Accrued interest payable	475	475	475	427	427	427
Derivative liabilities	35	35	35	38	38	38
Other liabilities	199	$ 199	$ 199	201	$ 201	$ 201
Variable Interest Entity, Initial Consolidation, Gain (Loss)	4			37			$ 0
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities		1	6		3	9	0
Number of DeConsolidated Variable Interest Entities		4			0		0
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	2,167	$ 2,167	$ 2,167	1,967	$ 1,967	$ 1,967
Restricted Cash	246	246	246	17	17	17	$ 2
Assets	4,394	4,394	4,394	4,054	4,054	4,054
Liabilities	4,404	4,404	4,404	4,160	4,160	4,160
Loans, Unpaid principal balance	1,787	1,787	1,787	1,977	1,977	1,977
Long-term Debt, Fair Value	2,710	2,710	2,710	2,788	2,788	2,788
Long-term debt, Unpaid principal balance	2,952	2,952	2,952	3,064	3,064	3,064
Long-term debt	2,967	2,967	2,967	3,107	3,107	3,107
Financing Receivable, after Allowance for Credit Loss	1,663	1,663	1,663	1,829	1,829	1,829
Derivative assets	226	226	226	239	239	239
Other assets	92	92	92	2	2	2
Long-term Debt, at par less amortized discount	258	258	258	319	319	319
Derivative liabilities	1,197	1,197	1,197	1,048	1,048	1,048
Other liabilities	5	5	$ 5	5	5	$ 5
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity			4			5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	2,072	2,072	$ 2,072	1,828	1,828	$ 1,828
Restricted Cash	245	245	245	1	1	1
Assets	4,296	4,296	4,296	3,896	3,896	3,896
Liabilities	4,244	4,244	4,244	3,836	3,836	3,836
Long-term Debt, Fair Value	2,710	2,710	2,710	2,788	2,788	2,788
Long-term debt	2,808	2,808	2,808	2,788	2,788	2,788
Financing Receivable, after Allowance for Credit Loss	1,663	1,663	1,663	1,829	1,829	1,829
Derivative assets	226	226	226	239	239	239
Other assets	90	90	90	0	0	0
Long-term Debt, at par less amortized discount	99	99	99	0	0	0
Derivative liabilities	1,197	1,197	1,197	1,048	1,048	1,048
Other liabilities	4	4	$ 4	0	0	$ 0
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity			2			4
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	95	95	$ 95	139	139	$ 139
Restricted Cash	1	1	1	16	16	16
Assets	98	98	98	157	157	157
Liabilities	160	160	160	324	324	324
Long-term Debt, Fair Value	0	0	0	0	0	0
Long-term debt	159	159	159	319	319	319
Financing Receivable, after Allowance for Credit Loss	0	0	0	0	0	0
Derivative assets	0	0	0	0	0	0
Other assets	2	2	2	2	2	2
Long-term Debt, at par less amortized discount	159	159	159	319	319	319
Derivative liabilities	0	0	0	0	0	0
Other liabilities	1	1	1	5	5	5
Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	508	508	508	639	639	639
Corporate Debt Securities [Member]
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	785	785	785	659	659	659
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4	4	4	1	1	1
Available-for-sale Securities	745	745	745	598	598	598
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	2,072	2,072	2,072	1,828	1,828	1,828
Corporate Debt Securities [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	2,072	2,072	2,072	1,828	1,828	1,828
Corporate Debt Securities [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	0	0	0	0	0	0
Municipal Bonds [Member]
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	72	72	72	44	44	44
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1	1	1	0	0	0
Available-for-sale Securities	72	72	72	43	43	43
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	88	88	88	99	99	99
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7	7	7	1	1	1
Available-for-sale Securities	95	95	95	96	96	96
Municipal Bonds [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	0	0	0	0	0	0
Municipal Bonds [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 95	$ 95	$ 95	$ 96	$ 96	$ 96