Variable Interest Entities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2021 USD ($) Entity	Dec. 31, 2021 USD ($) Entity	Dec. 31, 2020 USD ($) Entity	Dec. 31, 2019 USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 2,140,000,000	$ 2,140,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	141,000,000	141,000,000	$ 149,000,000
Available-for-sale Securities	2,265,000,000	2,265,000,000
Restricted Cash	5,000,000	5,000,000	13,000,000	$ 55,000,000
Fair value of special purpose entities	114,000	114,000
Assets	12,303,000,000	12,303,000,000	13,220,000,000
Liabilities	11,187,000,000	11,187,000,000	12,074,000,000
Equity Method Investments			51,000,000
Other Investments	690,000,000	690,000,000	595,000,000
Deconsolidated VIE Assets			0
Long-term debt	2,230,000,000	2,230,000,000	2,739,000,000
Derivative assets	76,000,000	76,000,000	93,000,000
Other assets	68,000,000	68,000,000	68,000,000
Accrued interest payable	576,000,000	576,000,000	517,000,000
Derivative liabilities	95,000,000	95,000,000	114,000,000
Other liabilities	$ 133,000,000	$ 133,000,000	$ 135,000,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities		0	1	1
Number of consolidated Variable Interest Entities | Entity	6	6	6
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 3,455,000,000	$ 3,455,000,000	$ 3,354,000,000
Restricted Cash	2,000,000	2,000,000	2,000,000	$ 2,000,000
Assets	6,216,000,000	6,216,000,000	6,398,000,000
Liabilities	6,156,000,000	6,156,000,000	6,328,000,000
Loans, Unpaid principal balance	2,363,000,000	2,363,000,000	2,546,000,000
Long-term Debt, Fair Value	4,056,000,000	4,056,000,000	4,324,000,000
Long-term debt, Unpaid principal balance	3,579,000,000	3,579,000,000	3,769,000,000
Long-term debt	4,216,000,000	4,216,000,000	4,493,000,000
Financing Receivable, after Allowance for Credit Loss	2,718,000,000	2,718,000,000	2,998,000,000
Derivative assets	38,000,000	38,000,000	41,000,000
Other assets	2,000,000	2,000,000	2,000,000
Long-term Debt, at par less amortized discount	160,000,000	160,000,000	169,000,000
Derivative liabilities	$ 1,940,000,000	$ 1,940,000,000	$ 1,835,000,000
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	5	5	5
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 3,320,000,000	$ 3,320,000,000	$ 3,215,000,000
Restricted Cash	1,000,000	1,000,000	1,000,000
Assets	6,077,000,000	6,077,000,000	6,255,000,000
Liabilities	5,996,000,000	5,996,000,000	6,159,000,000
Long-term Debt, Fair Value	4,056,000,000	4,056,000,000	4,324,000,000
Long-term debt	4,056,000,000	4,056,000,000	4,324,000,000
Financing Receivable, after Allowance for Credit Loss	2,718,000,000	2,718,000,000	2,998,000,000
Derivative assets	38,000,000	38,000,000	41,000,000
Other assets	0	0	0
Long-term Debt, at par less amortized discount	0	0	0
Derivative liabilities	$ 1,940,000,000	$ 1,940,000,000	$ 1,835,000,000
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	1	1	1
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 136,000,000	$ 136,000,000	$ 139,000,000
Restricted Cash	1,000,000	1,000,000	1,000,000
Assets	139,000,000	139,000,000	143,000,000
Liabilities	160,000,000	160,000,000	169,000,000
Long-term Debt, Fair Value	0	0	0
Long-term debt	160,000,000	160,000,000	169,000,000
Financing Receivable, after Allowance for Credit Loss	0	0	0
Derivative assets	0	0	0
Other assets	2,000,000	2,000,000	2,000,000
Long-term Debt, at par less amortized discount	160,000,000	160,000,000	169,000,000
Derivative liabilities	0	0	0
Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	2,230,000,000	2,230,000,000	2,739,000,000
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 0	0	247,000,000
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	$ 3,320,000,000	3,320,000,000	3,215,000,000
Corporate Debt Securities [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	3,320,000,000	3,320,000,000	3,215,000,000
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
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	29,000,000	29,000,000	27,000,000
Available-for-sale Securities	136,000,000	136,000,000	139,000,000
Municipal Bonds [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	0	0	0
Municipal Bonds [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 136,000,000	$ 136,000,000	$ 139,000,000