Fair Value Of Financial Instruments (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net insured derivative liability	$ 3,000,000,000		$ 2,900,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	3,200,000,000		4,400,000,000
Percentage of level 3 assets at fair value in total assets measured at fair value	29.00%		29.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value	74.00%		73.00%
Transfers into Level 3	85,000,000	28,000,000
Transfers out of Level 3	78,000,000	42,000,000
Transfers into Level 2	78,000,000	42,000,000
Transfers out of Level 2	85,000,000	28,000,000
Transfers into or out of Level 1	$ 0
Binomial Expansion Technique [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	80.00%
Direct Price Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	20.00%
Sector-Specific Spreads [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	8.00%
Corporate Spreads [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	42.00%
Spreads Benchmarked Relevant Spread Source [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	50.00%
WARF-Sourced [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	77.00%