Fair Value Of Financial Instruments (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 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	9.00%		9.00%
Net insured derivative liability	$ 144		$ 144		$ 244
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	$ 51		$ 51		$ 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	$ 23	$ 20	$ 23	$ 180
Transfers out of Level 3	24	68	36	82
Transfers into Level 2	24	68	36	82
Transfers out of Level 2	23	20	$ 23	$ 180
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	91.00%		91.00%