Variable Interest Entities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($)	Dec. 31, 2022 USD ($) Entity	Dec. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2021 USD ($) Entity	Dec. 31, 2020 USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 2,041,000,000	$ 2,041,000,000	$ 2,041,000,000	$ 2,041,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	31,000,000	31,000,000	31,000,000	31,000,000	$ 141,000,000	$ 141,000,000	$ 141,000,000
Available-for-sale Securities	1,966,000,000	1,966,000,000	1,966,000,000	1,966,000,000
Restricted Cash	14,000,000	14,000,000	14,000,000	14,000,000	5,000,000	5,000,000	5,000,000	$ 13,000,000
Fair value of special purpose entities	0	0	0	0
Assets	7,973,000,000	7,973,000,000	7,973,000,000	7,973,000,000	12,303,000,000	12,303,000,000	12,303,000,000
Liabilities	6,647,000,000	6,647,000,000	6,647,000,000	6,647,000,000	11,187,000,000	11,187,000,000	11,187,000,000
Other Investments	568,000,000	568,000,000	568,000,000	568,000,000	690,000,000	690,000,000	690,000,000
Long-term debt	639,000,000	639,000,000	639,000,000	639,000,000	2,230,000,000	2,230,000,000	2,230,000,000
Derivative assets	27,000,000	27,000,000	27,000,000	27,000,000	76,000,000	76,000,000	76,000,000
Other assets	84,000,000	84,000,000	84,000,000	84,000,000	68,000,000	68,000,000	68,000,000
Accrued interest payable	427,000,000	427,000,000	427,000,000	427,000,000	576,000,000	576,000,000	576,000,000
Derivative liabilities	38,000,000	38,000,000	38,000,000	38,000,000	95,000,000	95,000,000	95,000,000
Other liabilities	163,000,000	163,000,000	$ 163,000,000	$ 163,000,000	133,000,000	$ 133,000,000	$ 133,000,000
Variable Interest Entity, Initial Consolidation, Gain (Loss)		37,000,000			0			$ 0
Number of DeConsolidated Variable Interest Entities			0			0		0
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities			3	9		0	6
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	1,967,000,000	1,967,000,000	$ 1,967,000,000	$ 1,967,000,000	3,455,000,000	$ 3,455,000,000	$ 3,455,000,000
Restricted Cash	17,000,000	17,000,000	17,000,000	17,000,000	2,000,000	2,000,000	2,000,000	$ 2,000,000
Assets	4,054,000,000	4,054,000,000	4,054,000,000	4,054,000,000	6,216,000,000	6,216,000,000	6,216,000,000
Liabilities	4,160,000,000	4,160,000,000	4,160,000,000	4,160,000,000	6,156,000,000	6,156,000,000	6,156,000,000
Loans, Unpaid principal balance	1,977,000,000	1,977,000,000	1,977,000,000	1,977,000,000	2,363,000,000	2,363,000,000	2,363,000,000
Long-term Debt, Fair Value	2,788,000,000	2,788,000,000	2,788,000,000	2,788,000,000	4,056,000,000	4,056,000,000	4,056,000,000
Long-term debt, Unpaid principal balance	3,064,000,000	3,064,000,000	3,064,000,000	3,064,000,000	3,579,000,000	3,579,000,000	3,579,000,000
Long-term debt	3,107,000,000	3,107,000,000	3,107,000,000	3,107,000,000	4,216,000,000	4,216,000,000	4,216,000,000
Financing Receivable, after Allowance for Credit Loss	1,829,000,000	1,829,000,000	1,829,000,000	1,829,000,000	2,718,000,000	2,718,000,000	2,718,000,000
Derivative assets	239,000,000	239,000,000	239,000,000	239,000,000	38,000,000	38,000,000	38,000,000
Other assets	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000
Long-term Debt, at par less amortized discount	319,000,000	319,000,000	319,000,000	319,000,000	160,000,000	160,000,000	160,000,000
Derivative liabilities	1,048,000,000	1,048,000,000	1,048,000,000	1,048,000,000	1,940,000,000	1,940,000,000	1,940,000,000
Other liabilities	5,000,000	5,000,000	5,000,000	$ 5,000,000	0	0	$ 0
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity				5			5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	1,828,000,000	1,828,000,000	1,828,000,000	$ 1,828,000,000	3,320,000,000	3,320,000,000	$ 3,320,000,000
Restricted Cash	1,000,000	1,000,000	1,000,000	1,000,000	1,000,000	1,000,000	1,000,000
Assets	3,896,000,000	3,896,000,000	3,896,000,000	3,896,000,000	6,077,000,000	6,077,000,000	6,077,000,000
Liabilities	3,836,000,000	3,836,000,000	3,836,000,000	3,836,000,000	5,996,000,000	5,996,000,000	5,996,000,000
Long-term Debt, Fair Value	2,788,000,000	2,788,000,000	2,788,000,000	2,788,000,000	4,056,000,000	4,056,000,000	4,056,000,000
Long-term debt	2,788,000,000	2,788,000,000	2,788,000,000	2,788,000,000	4,056,000,000	4,056,000,000	4,056,000,000
Financing Receivable, after Allowance for Credit Loss	1,829,000,000	1,829,000,000	1,829,000,000	1,829,000,000	2,718,000,000	2,718,000,000	2,718,000,000
Derivative assets	239,000,000	239,000,000	239,000,000	239,000,000	38,000,000	38,000,000	38,000,000
Other assets	0	0	0	0	0	0	0
Long-term Debt, at par less amortized discount	0	0	0	0	0	0	0
Derivative liabilities	1,048,000,000	1,048,000,000	1,048,000,000	1,048,000,000	1,940,000,000	1,940,000,000	1,940,000,000
Other liabilities	0	0	0	$ 0	0	0	$ 0
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity				4			1
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	139,000,000	139,000,000	139,000,000	$ 139,000,000	136,000,000	136,000,000	$ 136,000,000
Restricted Cash	16,000,000	16,000,000	16,000,000	16,000,000	1,000,000	1,000,000	1,000,000
Assets	157,000,000	157,000,000	157,000,000	157,000,000	139,000,000	139,000,000	139,000,000
Liabilities	324,000,000	324,000,000	324,000,000	324,000,000	160,000,000	160,000,000	160,000,000
Long-term Debt, Fair Value	0	0	0	0	0	0	0
Long-term debt	319,000,000	319,000,000	319,000,000	319,000,000	160,000,000	160,000,000	160,000,000
Financing Receivable, after Allowance for Credit Loss	0	0	0	0	0	0	0
Derivative assets	0	0	0	0	0	0	0
Other assets	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000	2,000,000
Long-term Debt, at par less amortized discount	319,000,000	319,000,000	319,000,000	319,000,000	160,000,000	160,000,000	160,000,000
Derivative liabilities	0	0	0	0	0	0	0
Other liabilities	5,000,000	5,000,000	5,000,000	5,000,000	0	0	0
Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 639,000,000	639,000,000	639,000,000	639,000,000	2,230,000,000	2,230,000,000	2,230,000,000
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Notes
Variable Interest Entities [Line Items]
Note Holders	1
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 1,828,000,000	1,828,000,000	1,828,000,000	1,828,000,000	3,320,000,000	3,320,000,000	3,320,000,000
Corporate Debt Securities [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	1,828,000,000	1,828,000,000	1,828,000,000	1,828,000,000	3,320,000,000	3,320,000,000	3,320,000,000
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	0
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,000,000	1,000,000	1,000,000	1,000,000	29,000,000	29,000,000	29,000,000
Available-for-sale Securities	96,000,000	96,000,000	96,000,000	96,000,000	136,000,000	136,000,000	136,000,000
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	0
Municipal Bonds [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 96,000,000	$ 96,000,000	$ 96,000,000	$ 96,000,000	$ 136,000,000	$ 136,000,000	$ 136,000,000