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Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 46,927,000 | $ 62,716,000
Public Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 23,442,000 | 32,088,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 7,565,000 | 11,893,000
Public Finance [Member] | General Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 4,214,000 | 6,257,000
Public Finance [Member] | Utility Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,178,000 | 2,212,000
Public Finance [Member] | Higher Education [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,168,000 | 1,642,000
Public Finance [Member] | Housing Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,159,000 | 6,312,000
Public Finance [Member] | Health Care Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 459,000 | 807,000
Public Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 945,000 | 963,000
Public Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,754,000 | 2,002,000
Structured Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 9,947,000 | 13,816,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,510,000 | 7,267,000
Structured Finance [Member] | Investor-owned Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,754,000 | 3,274,000
Structured Finance [Member] | Structured Insurance [Domain] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,365,000 | 1,420,000
Structured Finance [Member] | Student Loan [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 934,000 | 1,238,000
Structured Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 237,000 | 443,000
Structured Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 147,000 | 174,000
International Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 13,538,000 | 16,812,000
International Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 0 | 246,000
International Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,550,000 | 2,609,000
International Finance [Member] | Investor-owned and Public Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 4,499,000 | 5,696,000
International Finance [Member] | Sovereign/Sub-sovereign [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,250,000 | 5,664,000
International Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,613,000 | 1,777,000
International Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 626,000 | $ 820,000

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