Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 28,020	$ 33,888
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,360	15,497
Public Finance [Member] | Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,196	4,179
Public Finance [Member] | General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,612	2,345
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,610	5,855
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,942	3,118
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,904	6,337
Structured Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,724	3,635
Structured Finance [Member] | Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,556	1,617
Structured Finance [Member] | Other Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	624	1,085
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	10,756	12,054
International Finance [Member] | Asset Backed and Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,276	1,374
International Finance [Member] | Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,283	3,899
International Finance [Member] | Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,141	5,270
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 1,056	$ 1,511