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Investment Securities - Narrative (Details)
3 Months Ended
Mar. 31, 2021
USD ($)
security
Mar. 31, 2020
USD ($)
Dec. 31, 2020
USD ($)
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]      
Investment securities held to maturity purchased $ 201,000,000 $ 29,000,000,000 $ 3,299,000,000
Debt securities, held-to-maturity, allowance for credit loss 2,000,000    
Pledged securities not separately reported $ 71,700,000,000   $ 70,570,000,000
Percentage of investment portfolio considered investment grade 99.00%    
Credit loss expense (reversal) $ (9,000,000) 36,000,000  
HTM securities writeoff 0    
Gross pre-tax unrealized losses $ 562,000,000    
Number of securities in loss position | security 664    
Held-to-Maturity Securities      
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]      
Credit loss expense (reversal) $ (1,000,000) $ 4,000,000