Financial Guarantees in Force - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Financial Guarantee Insurance Contracts, Accelerated Premium Revenue, Amount	$ 10	$ 32	$ 64
Gross par amount of financial guarantees outstanding	43,908	52,055
Net par amount of financial guarantees	38,018	46,927
Gross financial guarantees in force	69,826	87,543
Net financial guarantees in force	$ 58,245	$ 77,972
Highest single insured risk of aggregate net par amount guaranteed		2.70%
California [Member]
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	6.70%
NEW JERSEY
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	6.30%
NEW YORK
Guarantor Obligations [Line Items]
Aggregate net par amounts in force	6.10%
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	$ 17,653	$ 23,442
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	5,991	6,159
Military Housing [Domain] | Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net par amount of financial guarantees	$ 5,654	$ 5,759