Fair Value Of Financial Instruments (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net insured derivative liability	$ 2,900,000,000	$ 4,800,000,000
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	4,400,000,000	5,700,000,000
Interest rate on the surplus notes	14.00%
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	73.00%	77.00%
Transfers into Level 3	293,000,000	456,000,000
Transfers out of Level 3	157,000,000	391,000,000
Transfers into Level 2	157,000,000	391,000,000
Transfers out of Level 2	293,000,000	456,000,000
Transfers into or out of Level 1	$ 0	$ 0
Binomial Expansion Technique [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	82.00%
Percentage of sector specific transactions	9.00%
Corporate Spreads [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	46.00%
Spreads Benchmarked Relevant Spread Source [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	45.00%
WARF-Sourced [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	79.00%
Direct Price Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	18.00%