Fair Value Of Financial Instruments (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	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%
Net insured derivative liability	$ 85	$ 244
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	$ 99	$ 92
Percentage of level 3 assets at fair value in total assets measured at fair value value	19.00%	20.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value	52.00%	35.00%
Transfers into Level 3	$ 43	$ 240
Transfers out of Level 3	126	140
Transfers into Level 2	126	140
Transfers out of Level 2	43	240
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%