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	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unrealized gains (losses) on insured derivatives			$ 32	$ (16)
Net gains (losses) on financial instruments at fair value and foreign exchange	$ 22	$ (61)	13	(44)
Other net realized gains (losses)	0	34	(1)	37
Net investment losses related to other-than-temporary impairments	(1)	(11)	(2)	(13)
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	18	6	32	(16)
Realized gains and other settlements on insured derivatives	(25)	(3)	(44)	(34)
Net gains (losses) on financial instruments at fair value and foreign exchange	12	(37)	6	(40)
Other net realized gains (losses)	0		(2)
Net investment losses related to other-than-temporary impairments				0
Total revenues	15	2	18	(90)
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	18	6	32	(16)
Net gains (losses) on financial instruments at fair value and foreign exchange	12	(37)	6	(40)
Other net realized gains (losses)	0		(2)
Total revenues	44	5	63	(56)
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	10	36	26	0
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	$ 14	$ 36	$ 27	$ 0