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Investment gains/losses (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Gain (Loss) on Securities [Line Items]      
Equity securities - Unrealized investment gains/losses on securities held at the end of the period $ 69,581 $ (22,729)  
Equity securities - Investment gains/losses during the year on securities sold 1,585 291  
Equity securities - Investment gains/losses, total 71,166 (22,438) $ 1,318
Fixed maturity securities - Gross realized gains 87 480 103
Fixed maturity securities - Gross realized losses (25) (227) (22)
Other (105) 30 11
Investment gains/losses $ 71,123 $ (22,155) 1,410
Equity Securities [Member]      
Gain (Loss) on Securities [Line Items]      
Gross realized gains     2,237
Gross realized losses     $ (919)