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Investment Securities - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Jun. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Schedule of Held-to-maturity Securities [Line Items]        
Amount of investment securities transferred from AFS to HTM for capital management purposes $ 104,500      
Approximate percentage rated at least AA+   98.00% 98.00%  
Investment securities, allowance for credit losses   $ 73 $ 78 $ 120
Unrealized gains/(losses) on investment securities        
Schedule of Held-to-maturity Securities [Line Items]        
Pretax unrealized gains included in AOCI on the securities at the date of transfer $ 425