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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]      
Unrealized gains (losses) on insured derivatives $ 0 $ 21 $ 157
Net gains (losses) on financial instruments at fair value and foreign exchange (24) 84 63
Other net realized gains (losses) 31 (282) 17
Net investment losses related to other-than-temporary impairments (106) (5) (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 1 21 159
Realized gains and other settlements on insured derivatives (51) (43) (28)
Net gains (losses) on financial instruments at fair value and foreign exchange (32) 1 39
Net investment losses related to other-than-temporary impairments 0 (1)  
Total revenues 49 (8) 193
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 (1) 13 145
Net gains (losses) on financial instruments at fair value and foreign exchange (32) 1 42
Total revenues 98 46 206
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 131 14 23
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 $ 131 $ 32 $ 19