Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 144,734,000	$ 179,092,000
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	93,448,000	116,062,000
Public Finance [Member] | Lease and Tax-backed Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	33,411,000	41,858,000
Public Finance [Member] | General Obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	22,699,000	29,115,000
Public Finance [Member] | Utility Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	11,687,000	14,933,000
Public Finance [Member] | Transportation Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,738,000	9,653,000
Public Finance [Member] | Higher Education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,389,000	7,810,000
Public Finance [Member] | Housing Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,108,000	7,439,000
Public Finance [Member] | Health Care Revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,106,000	3,903,000
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,310,000	1,351,000
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	26,334,000	31,412,000
Structured Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	13,686,000	16,026,000
Structured Finance [Member] | Investor-owned Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,411,000	5,881,000
Structured Finance [Member] | Student Loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,390,000	4,357,000
Structured Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,335,000	2,361,000
Structured Finance [Member] | Collateralized Debt Obligations [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	637,000	897,000
Structured Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,875,000	1,890,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	24,952,000	31,618,000
International Finance [Member] | Mortgage-backed and Home Equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	410,000	484,000
International Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	4,442,000	6,884,000
International Finance [Member] | Collateralized Debt Obligations [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	233,000	822,000
International Finance [Member] | Investor-owned and Public Utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	8,455,000	9,595,000
International Finance [Member] | Sovereign/Sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,758,000	7,394,000
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,425,000	5,021,000
International Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 1,229,000	$ 1,418,000