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Securities - Realized gains and losses on investment securities Transactions Attributable to available-for-sale securities (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]        
Gross realized gains $ 53 $ 64 $ 140 $ 107
Gross realized losses (1) (18) (29) (31)
Net realized gains $ 52 $ 46 $ 111 $ 76