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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Net Derivative Contract Liabilities [Member]    
Fair value measurements Significant unobservable inputs [Line Items]    
Beginning Balance $ (4,759) $ (5,255)
Gains (losses) included in earnings 467 644
Gains (losses) included in other comprehensive income (5) 4
Gains (losses) included in regulatory assets and liabilities (21) (3)
Acquisitions, dispositions and settlements (65)  
Dispositions and settlements   (1)
Transfers into/out of Level 3 3 (35)
Ending Balance (4,380) (4,646)
Fixed Maturities [Member]    
Fair value measurements Significant unobservable inputs [Line Items]    
Beginning Balance 8 372
Gains (losses) included in other comprehensive income   13
Dispositions and settlements   (1)
Acquisitions, dispositions and settlements 103  
Transfers into/out of Level 3   (375)
Ending Balance 111 9
Equity Securities and Other Investments [Member]    
Fair value measurements Significant unobservable inputs [Line Items]    
Beginning Balance 21,996 17,958
Gains (losses) included in other comprehensive income (1,722) 1,228
Transfers into/out of Level 3   (6)
Ending Balance $ 20,274 $ 19,180