Fair Value Of Financial Instruments (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, value under Direct Price Model and Dual-Default Model	12.00%		12.00%
Net insured derivative liability	$ 121		$ 121		$ 244
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	$ 42		$ 42		$ 92
Percentage of level 3 assets at fair value in total assets measured at fair value value	20.00%		20.00%		20.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value	53.00%		53.00%		35.00%
Transfers into Level 3	$ 3	$ 55	$ 34	$ 238
Transfers out of Level 3	77	1	122	83
Transfers into Level 2	77	1	122	83
Transfers out of Level 2	3	55	$ 34	$ 238
Transfers into or out of Level 1	$ 0	$ 0
B E T 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	88.00%		88.00%