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