Quarterly report pursuant to Section 13 or 15(d)

Changes in Level 3 Instruments Measured on a Recurring Basis (Detail)

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Changes in Level 3 Instruments Measured on a Recurring Basis (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Mar. 31, 2012
Mar. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]            
Beginning Balance     $ 217 $ 405 $ 190 $ 240
Transfers into (out of) Level 3 0 0 0 0    
Net realized and unrealized gains (losses):            
Included in earnings (in investment income) (13) (12) (40) (39)    
Included in accumulated other comprehensive income (pre-tax) 0 0 0 0    
Purchases 0 0 0 0    
Sales 0 0 0 (138)    
Ending Balance 177 228 177 228 190 240
Marketable securities
           
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]            
Beginning Balance     0 138 0 0
Transfers into (out of) Level 3 0 0 0 0    
Net realized and unrealized gains (losses):            
Included in earnings (in investment income) 0 0 0 0    
Included in accumulated other comprehensive income (pre-tax) 0 0 0 0    
Purchases 0 0 0 0    
Sales 0 0 0 (138)    
Ending Balance 0 0 0 0 0 0
Other Long-term Investments
           
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]            
Beginning Balance     217 267 190 240
Transfers into (out of) Level 3 0 0 0 0    
Net realized and unrealized gains (losses):            
Included in earnings (in investment income) (13) (12) (40) (39)    
Included in accumulated other comprehensive income (pre-tax) 0 0 0 0    
Purchases 0 0 0 0    
Sales 0 0 0 0    
Ending Balance $ 177 $ 228 $ 177 $ 228 $ 190 $ 240