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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, 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

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