Variable Interest Entities - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Dec. 31, 2020 USD ($) Entity	Dec. 31, 2020 USD ($) Entity	Dec. 31, 2019 USD ($) Entity	Dec. 31, 2018 USD ($)	Feb. 12, 2018 USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 2,807,000,000	$ 2,807,000,000	$ 3,187,000,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	149,000,000	149,000,000	132,000,000
satisfaction and discharge of segregated account deferred amounts					$ 3,000,000,000
satisfaction and discharge of segregated account deferred interest amounts					857,000,000
Available-for-sale Securities	2,949,000,000	2,949,000,000	3,314,000,000
Restricted Cash	13,000,000	13,000,000	55,000,000	$ 19,000,000
Fair value of special purpose entities	1,000,000	1,000,000	3,000,000
Variable Interest Entity, Consolidated, Carrying Amount, Assets	292,000,000	292,000,000
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	364,000,000	364,000,000
Equity Method Investments	51,000,000	51,000,000	46,000,000
Other Investments	595,000,000	595,000,000	478,000,000
Deconsolidated VIE Assets	0	0
Deconsolidated VIE Liabilities	0	0
Long-term debt	2,739,000,000	2,739,000,000	2,822,000,000
Derivative assets	93,000,000	93,000,000	75,000,000
Other assets	114,000,000	114,000,000	95,000,000
Assets	13,220,000,000	13,220,000,000	13,320,000,000
Accrued interest payable	517,000,000	517,000,000	441,000,000
Derivative liabilities	114,000,000	114,000,000	90,000,000
Other liabilities	106,000,000	106,000,000	93,000,000
Liabilities	12,074,000,000	$ 12,074,000,000	11,783,000,000
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life		2 months 12 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	$ 410,000,000	$ 410,000,000	$ 403,000,000
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities		1	1	4
Number of consolidated Variable Interest Entities | Entity	6	6	7
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	5	5	6
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Restricted Cash	$ 9,000,000	$ 9,000,000	$ 55,000,000
Secured Notes Received from Ambac LSNI					644,000,000
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	1	1	1
AFG [Member]
Variable Interest Entities [Line Items]
Secured Notes Received from Ambac LSNI					$ 125,000,000
Ambac Assurance Corporation [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 2,739,000,000	$ 2,739,000,000	$ 2,822,000,000
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member]
Variable Interest Entities [Line Items]
Long-term debt	$ 247,000,000	$ 247,000,000	$ 252,000,000
Ambac Assurance Corporation [Member] | 5.1% Junior Surplus Notes, General Account Due 2020 [Member] | Corolla Notes
Variable Interest Entities [Line Items]
Note Holders	1