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Investment Securities Available for Sale (Narrative) (Details)
Sep. 30, 2021
USD ($)
security
Dec. 31, 2020
USD ($)
Financing Receivable, Impaired [Line Items]    
Investment securities in a loss position for less than twelve months | security 135  
Investment securities in a loss position for twelve months or longer | security 15  
Allowance for credit loss | $ $ 0  
Collateral Pledged    
Financing Receivable, Impaired [Line Items]    
Pledged to secure deposits | $ $ 3,740,000,000 $ 3,840,000,000