Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 79,346,000	$ 108,299,000
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	45,062,000	65,436,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	15,688,000	22,060,000
Public Finance [Member] | General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	9,867,000	15,946,000
Public Finance [Member] | Utility Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,298,000	8,218,000
Public Finance [Member] | Higher Education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,339,000	3,439,000
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,508,000	6,810,000
Public Finance [Member] | Health Care Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,484,000	2,234,000
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,018,000	1,140,000
Public Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,860,000	5,589,000
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	16,951,000	21,814,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	9,383,000	11,387,000
Structured Finance [Member] | Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,833,000	4,921,000
Structured Finance [Member] | Student Loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,388,000	2,323,000
Structured Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	565,000	1,140,000
Structured Finance [Member] | Collateralized Debt Obligations [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	132,000	306,000
Structured Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,650,000	1,737,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	17,333,000	21,049,000
International Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	254,000	347,000
International Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,951,000	3,870,000
International Finance [Member] | Collateralized Debt Obligations [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	186,000	190,000
International Finance [Member] | Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,168,000	7,208,000
International Finance [Member] | Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,211,000	6,218,000
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,700,000	2,118,000
International Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 863,000	$ 1,098,000