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Summary of investment gains and losses (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Gain (Loss) on Securities [Line Items]        
Equity securities - Unrealized investment gains/losses on securities held at the end of the period $ 14,294   $ 12,126  
Equity securities - Investment gains/losses during 2018 on securities sold in 2018 244   307  
Investment gains (losses) 14,569 $ 657 12,750 $ 1,262
Fixed maturity securities - Gross realized gains 44 56 451 82
Fixed maturity securities - Gross realized losses (10) (2) (152) (16)
Equity Securities [Member]        
Gain (Loss) on Securities [Line Items]        
Gross realized gains   1,011   1,795
Gross realized losses   (419)   (626)
Investment gains (losses) 14,538 592 12,433 1,169
Other [Member]        
Gain (Loss) on Securities [Line Items]        
Investment gains (losses) $ (3) $ 11 $ 18 $ 27