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Fair value measurements - Significant unobservable inputs (Detail) - Significant Unobservable Inputs (Level 3) [Member] - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2015
Jun. 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 1,200 369
Gains (losses) included in other comprehensive income (3) 4
Gains (losses) included in regulatory assets and liabilities (17)  
Dispositions and settlements   1
Acquisitions, dispositions and settlements (51)  
Transfers into/out of Level 3 3 (35)
Ending Balance (3,627) (4,916)
Fixed Maturities [Member]    
Fair value measurements Significant unobservable inputs [Line Items]    
Beginning Balance 8 372
Dispositions and settlements   (1)
Acquisitions, dispositions and settlements (1)  
Transfers into/out of Level 3   (56)
Ending Balance 7 315
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 (329) (31)
Ending Balance $ 21,667 $ 17,927