Fair Value Of Financial Instruments (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net insured derivative liability	$ 3,300,000,000		$ 3,300,000,000		$ 4,800,000,000
Impact of excluding the nonperformance risk in valuation of pre-tax net insured derivative liability	4,400,000,000		4,400,000,000		5,700,000,000
Percentage of level 3 assets at fair in total assets measured at fair value			30.00%		29.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value			74.00%		77.00%
Transfers into Level 3	168,000,000	144,000,000	270,000,000	350,000,000
Transfers out of Level 3	44,000,000	77,000,000	99,000,000	354,000,000
Transfers into Level 2	44,000,000	77,000,000	99,000,000	354,000,000
Transfers out of Level 2	168,000,000	144,000,000	270,000,000	350,000,000
Transfers into or out of Level 1	$ 0	$ 0	$ 0	$ 0
Binomial Expansion Technique [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	81.00%		81.00%
Percentage of sector specific transactions	9.00%		9.00%
Direct Price Model [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives	19.00%		19.00%
Corporate Spreads [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	46.00%		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%		45.00%
WARF Sourced [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of sector specific transactions	77.00%		77.00%