Fair Value Of Financial Instruments (Realized And Unrealized Gains And Losses Included In Earnings Pertaining To Level 3 Assets And Liabilities) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains (losses) on insured derivatives	$ 21	$ 157	$ 903
Net gains (losses) on financial instruments at fair value and foreign exchange	84	63	78
Net investment losses related to other-than-temporary impairments	(5)	(13)	(15)
Fair Value Inputs Level 3 [Member] | Total Gains Losses Included In Earnings [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains (losses) on insured derivatives	21	159	903
Realized gains and other settlements on insured derivatives	(43)	(28)	(445)
Net gains (losses) on financial instruments at fair value and foreign exchange	1	39	18
Net investment losses related to other-than-temporary impairments	(1)
Total revenues	(8)	193	444
Fair Value Inputs Level 3 [Member] | Change In Unrealized Gains Losses For Period Included In Earnings For Assets And Liabilities Still Held [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains (losses) on insured derivatives	13	145	20
Net gains (losses) on financial instruments at fair value and foreign exchange	1	42	12
Total revenues	46	206	52
Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Total Gains Losses Included In Earnings [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	14	23	(32)
Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Change In Unrealized Gains Losses For Period Included In Earnings For Assets And Liabilities Still Held [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Net gains (losses) on financial instruments at fair value and foreign exchange	$ 32	$ 19	$ 20