Variable Interest Entities - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Entity	Dec. 31, 2018 USD ($) Entity	Dec. 31, 2017 USD ($)	Feb. 12, 2018 USD ($)
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	$ 3,187	$ 3,451
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	132	133
satisfaction and discharge of segregated account deferred amounts				$ 3,000
satisfaction and discharge of segregated account deferred interest amounts				857
Available-for-sale Securities	3,314
Restricted Cash	55	19
Fair value of special purpose entities	3	5
Variable Interest Entity, Consolidated, Carrying Amount, Assets	292
Variable Interest Entity, Consolidated, Carrying Amount, Liabilities	364
Equity Method Investments	46	40
Loans, Unpaid principal balance		19	$ 19
Other Investments	478	391
Deconsolidated VIE Assets	1,233
Deconsolidated VIE Liabilities	1,230
Long-term debt	2,822	2,929
Derivative assets	75	59
Other assets	95	138
Assets	13,320	14,589
Accrued interest payable	441	376
Derivative liabilities	90	77
Other liabilities	93	64
Liabilities	$ 11,783	12,956
Asset-Backed Securities and Utility Obligations [Member]
Variable Interest Entities [Line Items]
Weighted average life	1 year 1 month 6 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	$ 403	393
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	3,121	2,737
Restricted Cash	2	1	$ 1
Long-term Debt, Fair Value	4,351	5,269
Long-term debt	4,554	5,269
Financing Receivable, after Allowance for Credit Loss	3,108	4,288
Derivative assets	52	66
Other assets	3	1
Assets	6,286	7,093
Accrued interest payable	1	1
Long-term Debt, at par less amortized discount	203	0
Derivative liabilities	1,657	1,712
Liabilities	$ 6,212	$ 6,981
Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of DeConsolidated Variable Interest Entities	1	4	1
Number of consolidated Variable Interest Entities | Entity	7	7
Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 2,957	$ 2,737
Restricted Cash	1	1
Long-term Debt, Fair Value	4,351	5,269
Long-term debt	4,351	5,269
Financing Receivable, after Allowance for Credit Loss	3,108	4,288
Derivative assets	52	66
Other assets	1	1
Assets	6,119	7,093
Accrued interest payable	1	1
Long-term Debt, at par less amortized discount	0	0
Derivative liabilities	1,657	1,712
Liabilities	$ 6,009	$ 6,981
Ambac UK [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	6	7
Ambac Assurance [Member]
Variable Interest Entities [Line Items]
Secured Notes Received from Ambac LSNI				644
Unsettled Amounts Secured Notes from LSNI	$ 55	$ 19
Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	164	0
Restricted Cash	1	0
Long-term Debt, Fair Value	0	0
Long-term debt	203	0
Financing Receivable, after Allowance for Credit Loss	0	0
Derivative assets	0	0
Other assets	2	0
Assets	167	0
Accrued interest payable	0	0
Long-term Debt, at par less amortized discount	203	0
Derivative liabilities	0	0
Liabilities	$ 203	$ 0
Ambac Assurance [Member] | Consolidated Entities [Member]
Variable Interest Entities [Line Items]
Number of consolidated Variable Interest Entities | Entity	1	0
AFG [Member]
Variable Interest Entities [Line Items]
Secured Notes Received from Ambac LSNI				$ 125
Corporate Debt Securities [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 2,957	$ 2,737
Corporate Debt Securities [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	2,957	2,737
Corporate Debt Securities [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	0	0
Municipal Bonds [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Fixed income securities, amortized cost	139
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	25
Available-for-sale Securities	164	0
Municipal Bonds [Member] | Ambac UK [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	0	0
Municipal Bonds [Member] | Ambac Assurance [Member] | Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entities [Line Items]
Available-for-sale Securities	$ 164	$ 0