Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($) Entity	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) Entity	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 2,000,000	$ (2,000,000)	$ 1,000,000	$ (2,000,000)
Realized Investment Gains (Losses)	(2,000,000)	10,000,000	1,000,000	18,000,000
Other Operating Income (Expense), Net	7,000,000	0	12,000,000	0
Fair value of special purpose entities	114,000		114,000		$ 1,000,000
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	2,000,000	0	2,000,000	3,000,000
Long-term debt	$ 2,671,000,000		$ 2,671,000,000		2,739,000,000
Sponsored Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Total principal amount of debt outstanding					$ 410,000,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	6		6		6
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	$ 0	1,000,000	$ 1,000,000	(3,000,000)
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	2,000,000	(1,000,000)	1,000,000	(5,000,000)
Interest Expense, Long-term Debt	(1,000,000)	$ (1,000,000)	(3,000,000)	$ (3,000,000)
Long-term debt	$ 4,434,000,000		$ 4,434,000,000		$ 4,493,000,000
Consolidated Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities	0	1	0	1
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]
Long-term debt	$ 4,270,000,000		$ 4,270,000,000		$ 4,324,000,000
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	463,000,000		463,000,000		465,000,000
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	2,000,000	$ 2,000,000	3,000,000	$ 3,000,000
Realized Investment Gains (Losses)	0	$ 0	1,000,000	$ 8,000,000
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,626,000,000		$ 1,626,000,000		$ 1,641,000,000
5.1% Junior Surplus Notes, General Account Due 2020 | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Note Holders	1
5.1% Junior Surplus Notes, General Account Due 2020 | Ambac Assurance Corporation [Member] | Corolla Notes
Variable Interest Entities [Line Items]
Note Holders	1