Special Purpose Entities, Including Variable Interest Entities - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) Entity	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) Entity
Variable Interest Entities [Line Items]
Variable Interest Entity Change in Fair Value of Assets and Liabilities	$ 0	$ 2
Realized Investment Gains (Losses)	8	17
Equity Method Investments	47		$ 46
Deconsolidated VIE Assets		292
Deconsolidated VIE Liabilities		364
Fair value of special purpose entities	3		3
variable interest entities deconsolidation gain loss	0	15
Variable Interest Entity, Measure of Activity, Income or Loss before Tax	3	16
Other investments	363		478
Long-term debt	$ 2,760		2,822
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life	10 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	$ 378		403
Variable Interest Entities [Member]
Variable Interest Entities [Line Items]
Other Comprehensive Income (Loss), Financial Liability, Fair Value Option, Reclassification Adjustment from AOCI for Derecognition, before Tax	(4)	0
VIE Change in FV of Assets Liabilities, Net of Credit Risk Change	(4)	2
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Interest Expense, Long-term Debt	(2)	(2)
Other Expenses	0	$ 0
Long-term debt	$ 4,263		$ 4,554
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of Newly Consolidated Variable Interest Entities	0	0
Number of DeConsolidated Variable Interest Entities	0	0
Number Of Consolidated Variable Interest Entities | Entity	7		7
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities	0	0
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 4,092		$ 4,351
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number Of Consolidated Variable Interest Entities | Entity	6		6
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Carry Value Secured Notes from LSNI	$ 495		$ 535
Fixed Income Investments [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Gross Investment Income, Operating	2	$ 2
Realized Investment Gains (Losses)	8	$ 0
Ambac Note Post Exit From Rehabilitation [Member] | Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 1,685		$ 1,763