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Investment gains (losses) - Narrative (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Gain (Loss) on Securities [Line Items]        
Sales of equity securities     $ 24,046 $ 133,218
Taxable gains (losses) on sales of equity securities $ 10,400 $ 23,400 $ 18,700 $ 97,100