Financial Guarantees in Force - Summary of International Finance Guaranteed Portfolio (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 33,888	$ 38,018
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,054	12,857
International Finance [Member] | United Kingdom [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	9,711	10,593
International Finance [Member] | Australia [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	420	382
International Finance [Member] | Italy [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	803	767
International Finance [Member] | Austria [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	707	674
International Finance [Member] | FRANCE
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	277	303
International Finance [Member] | Other International [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 136	$ 138