Fair Value Measurement (Narrative) (Detail) (USD $)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector-specific transactions valued using BET model	9.00%		9.00%
Percentage of transactions valued using corporate spreads used	47.00%		47.00%
Percentage of transactions valued using spread benchmarked from most relevant spread source	44.00%		44.00%
Percentage of transactions valued using WARF-sourced and/or ratings sourced credit spreads	76.00%		76.00%
Net insured derivative liability	$ 3,300,000,000		$ 3,300,000,000		$ 4,800,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	4,300,000,000		4,300,000,000		5,700,000,000
Percentage of level 3 assets at fair in total assets measured at fair value value			28.00%		29.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value			73.00%		77.00%
Transfers into Level 3	72,000,000	166,000,000	103,000,000	208,000,000
Transfers out of Level 3	15,000,000	28,000,000	55,000,000	264,000,000
Transfers into Level 2	15,000,000	28,000,000	55,000,000	264,000,000
Transfers out of Level 2	72,000,000	166,000,000	103,000,000	208,000,000
Transfers into or out of Level 1	0		0
Net unrealized loss related to transfer into Level 3	8,000,000	2,000,000	17,000,000	1,000,000
Net unrealized losses on securities transferred out of Level 3	2,000,000		5,000,000
Net unrealized loss related to transfers out of Level 3		$ 8,000,000		$ 22,000,000
BET Valuation Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, fair value using the BET model	79.00%		79.00%
Direct Price Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, value under Direct Price Model	21.00%		21.00%

Approximate percentage of level 3 assets at fair value in total assets measured at fair value.

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- Definition

Approximate percentage of level 3 liabilities at fair value in total liabilities measured at fair value.

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- Definition

Impact Of Non Performance Risk On Insured Derivative Liability

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- Definition

Net Unrealized Gains Related to the Transfers Out of Level Three

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Net Unrealized Loss Related to Transfers Into Level Three

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Net Unrealized Loss Related to Transfers Out of Level Three

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- Definition

Percentage Of Insured Credit Derivatives Value Under Bet Model

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- Definition

Percentage of insured credit derivatives value under direct price model.

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- Definition

Percentage Of Sector Specific Transactions Valued Using BET Model

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- Definition

Percentage Of Transactions Valued Using Corporate Spread

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Percentage of Transactions Valued Using Spread Benchmark From Most Relevant Spread Source

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- Definition

Percentage of Transactions Valued Using Warf Sourced And Or Ratings Source Credit Spread

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- Definition

Transfers of assets measured at fair value on a recurring basis into and out of level one during the period.

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Transfers of assets measured at fair value on a recurring basis into level three during the period.

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No definition available.

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Transfers of assets measured at fair value on a recurring basis into level two during the period.

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- Definition

Transfers of assets measured at fair value on a recurring basis out of level three during the period.

+ References

No definition available.

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- Definition

Transfers of assets measured at fair value on a recurring basis out of level two during the period.

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 10

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 4, 17

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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