Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 12	$ 10	$ 32
Gross par amount of financial guarantees outstanding	39,070	43,908
Net par amount of financial guarantees	33,888	38,018
Gross financial guarantees in force	61,895	69,826
Net financial guarantees in force	$ 51,603	58,245
Highest single insured risk of aggregate net par amount guaranteed	2.90%
Ceded Principal Outstanding Major Reinsurer Percentage	6.10%
California [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	6.20%
NEW JERSEY
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	7.00%
NEW YORK
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.40%
No other state [Member] | Maximum [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	5.00%
Public Finance [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 15,497	17,653
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	5,855	5,991
Military Housing [Member] | Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 5,575	$ 5,654