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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, 2017 | Dec. 31, 2016
--- | --- | ---
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 62,716,000 | $ 79,346,000
Public Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 32,088,000 | 45,062,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 11,893,000 | 15,688,000
Public Finance [Member] | General Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,257,000 | 9,867,000
Public Finance [Member] | Utility Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,212,000 | 4,298,000
Public Finance [Member] | Higher Education [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,642,000 | 2,339,000
Public Finance [Member] | Housing Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,312,000 | 6,508,000
Public Finance [Member] | Health Care Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 807,000 | 1,484,000
Public Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 963,000 | 1,018,000
Public Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,002,000 | 3,860,000
Structured Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 13,816,000 | 16,951,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 7,267,000 | 9,383,000
Structured Finance [Member] | Investor-owned Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 3,274,000 | 3,833,000
Structured Finance [Member] | Student Loan [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,238,000 | 1,388,000
Structured Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 443,000 | 565,000
Structured Finance [Member] | Collateralized Debt Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 33,000 | 132,000
Structured Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,561,000 | 1,650,000
International Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 16,812,000 | 17,333,000
International Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 246,000 | 254,000
International Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,609,000 | 2,951,000
International Finance [Member] | Collateralized Debt Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 0 | 186,000
International Finance [Member] | Investor-owned and Public Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,696,000 | 6,168,000
International Finance [Member] | Sovereign/Sub-sovereign [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,664,000 | 5,211,000
International Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,777,000 | 1,700,000
International Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 820,000 | $ 863,000

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