Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 38,018	$ 46,927
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	17,653	23,442
Public Finance [Member] | Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,102	7,565
Public Finance [Member] | General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,011	4,214
Public Finance [Member] | Utility Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	768	1,178
Public Finance [Member] | Higher Education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	885	1,168
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,991	6,159
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,041	1,404
Public Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	855	1,754
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,508	9,947
Structured Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,423	5,510
Structured Finance [Member] | Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,675	1,754
Structured Finance [Member] | Structured Insurance [Domain]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	395	1,365
Structured Finance [Member] | Student Loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	769	934
Structured Finance [Member] | Asset Backed and Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	246	384
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	12,857	13,538
International Finance [Member] | Asset Backed and Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,625	2,176
International Finance [Member] | Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,436	4,499
International Finance [Member] | Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,264	5,250
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 1,532	$ 1,613