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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, 2015 | Dec. 31, 2014
--- | --- | ---
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 108,299,000 | $ 144,734,000
Public Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 65,436,000 | 93,448,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 22,060,000 | 33,411,000
Public Finance [Member] | General Obligation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 15,946,000 | 22,699,000
Public Finance [Member] | Utility Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 8,218,000 | 11,687,000
Public Finance [Member] | Higher Education [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 3,439,000 | 6,389,000
Public Finance [Member] | Housing Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,810,000 | 7,108,000
Public Finance [Member] | Health Care Revenue [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,234,000 | 3,106,000
Public Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,140,000 | 1,310,000
Public Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 5,589,000 | 7,738,000
Structured Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 21,814,000 | 26,334,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 11,387,000 | 13,686,000
Structured Finance [Member] | Investor-owned Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 4,921,000 | 5,411,000
Structured Finance [Member] | Student Loan [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,323,000 | 3,390,000
Structured Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,140,000 | 1,335,000
Structured Finance [Member] | Collateralized Debt Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 306,000 | 637,000
Structured Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 1,737,000 | 1,875,000
International Finance [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 21,049,000 | 24,952,000
International Finance [Member] | Mortgage-backed and Home Equity [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 347,000 | 410,000
International Finance [Member] | Asset-backed Securities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 3,870,000 | 4,442,000
International Finance [Member] | Collateralized Debt Obligations [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 190,000 | 233,000
International Finance [Member] | Investor-owned and Public Utilities [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 7,208,000 | 8,455,000
International Finance [Member] | Sovereign/Sub-sovereign [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 6,218,000 | 6,758,000
International Finance [Member] | Transportation [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | 2,118,000 | 3,425,000
International Finance [Member] | Other [Member] |  |
Guarantor Obligations [Line Items] |  |
Net Par Amount Outstanding | $ 1,098,000 | $ 1,229,000

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