Financial Guarantee Insurance - Summary of Percentage Ceded to Reinsurers and Reinsurance Recoverable and Rating Levels (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	100.00%
Net unsecured reinsurance recoverable	$ 34
Assured Guaranty Re Ltd [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	46.00%
Net unsecured reinsurance recoverable	$ 0
Sompo Japan Nipponkoa Holdings, Inc [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	3.00%
Net unsecured reinsurance recoverable	$ 0
Assured Guaranty Corporation [Member]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	8.00%
Net unsecured reinsurance recoverable	$ 4
Build America Mutual Assurance Company [Domain]
Summary of Ceded Par and Net Unsecured Reinsurance Recoverable [Line Items]
Percentage ceded Par	43.00%
Net unsecured reinsurance recoverable	$ 30