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Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 33,888 | $ 38,018
Public Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 15,497 | 17,653
Public Finance [Member] | Lease and Tax-backed Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 4,179 | 5,102
Public Finance [Member] | General Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,345 | 3,011
Public Finance [Member] | Utility Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 675 | 768
Public Finance [Member] | Higher Education [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 747 | 885
Public Finance [Member] | Housing Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,855 | 5,991
Public Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 925 | 1,041
Public Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 771 | 855
Structured Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,337 | 7,508
Structured Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 3,635 | 4,423
Structured Finance [Member] | Investor-owned Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,617 | 1,675
Structured Finance [Member] | Structured Insurance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 311 | 395
Structured Finance [Member] | Student Loan [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 626 | 769
Structured Finance [Member] | Asset Backed and Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 148 | 246
International Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 12,054 | 12,857
International Finance [Member] | Asset Backed and Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,374 | 1,625
International Finance [Member] | Investor-owned and Public Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 3,899 | 4,436
International Finance [Member] | Sovereign/Sub-sovereign [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,270 | 5,264
International Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 1,511 | $ 1,532

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