Financial Guarantees in Force - Summary of Financial Guarantee Portfolio Diversification by Type of Guaranteed Bond (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 223,633,000	$ 272,504,000
Public Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	143,018,000	176,817,000
Public Finance [Member] | Lease and tax-backed revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	50,415,000	59,864,000
Public Finance [Member] | General obligation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	34,623,000	42,959,000
Public Finance [Member] | Utility revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	18,651,000	22,529,000
Public Finance [Member] | Transportation revenues [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	13,892,000	18,945,000
Public Finance [Member] | Higher education [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	10,382,000	13,618,000
Public Finance [Member] | Housing revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	8,176,000	8,823,000
Public Finance [Member] | Health care revenue [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,266,000	7,824,000
Public Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,613,000	2,255,000
Structured Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	42,359,000	55,145,000
Structured Finance [Member] | Mortgage-backed and home equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	19,117,000	23,164,000
Structured Finance [Member] | Investor-owned utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,071,000	9,049,000
Structured Finance [Member] | Other CDOs [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,941,000	8,060,000
Structured Finance [Member] | Student loan [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	5,411,000	7,824,000
Structured Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	2,905,000	4,732,000
Structured Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,914,000	2,316,000
International Finance [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	38,256,000	40,542,000
International Finance [Member] | Mortgage-backed and home equity [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	1,268,000	1,397,000
International Finance [Member] | Other CDOs [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	3,191,000	4,375,000
International Finance [Member] | Asset-backed Securities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	8,702,000	9,560,000
International Finance [Member] | Investor-owned and public utilities [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	10,314,000	10,510,000
International Finance [Member] | Sovereign/sub-sovereign [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	7,289,000	7,282,000
International Finance [Member] | Transportation [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	6,002,000	5,914,000
International Finance [Member] | Other [Member]
Guarantor Obligations [Line Items]
Net Par Amount Outstanding	$ 1,490,000	$ 1,504,000