Special Purpose Entities, Including Variable Interest Entities - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($) Entity	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) Entity	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ (1)	$ 2	$ (3)	$ 7
Realized Investment Gains (Losses)	2	18	20	71
Other Operating Income (Expense), Net	2	141	2	133
Fair value of special purpose entities	2		2		$ 3
Gain (loss) from consolidating FG VIEs	0	0	0	15
Variable Interest Entity Deconsolidation Gain Loss	0	(2)	0	(2)
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	0	11	3	30
Long-term debt	2,737		2,737		2,822
Equity Method Investments	49		$ 49		46
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life			4 months 24 days
Average rating of assets held by sponsored special purpose entities			BBB+
Sponsored Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Total principal amount of debt outstanding	$ 392		$ 392		$ 403
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	6		6		7
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	0	$ (2)	0
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	0	2	(5)	6
Interest Expense, Long-term Debt	(1)	(3)	(5)	(9)
Other Operating Income (Expense), Net	0	$ 0	0	(1)
Long-term debt	$ 4,169		$ 4,169		$ 4,554
Consolidated Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities	0	1	1
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	5		5		6
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 4,002		$ 4,002		$ 4,351
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	470		470		535
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	2	$ 3	5	8
Realized Investment Gains (Losses)	0	$ 12	8	$ 13
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,648		$ 1,648		$ 1,763