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Investment Securities - Narrative (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
security
Jun. 30, 2019
USD ($)
Mar. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Investments, Debt and Equity Securities [Abstract]          
Investment securities held to maturity purchased $ 11,257,000,000 $ 11,257,000,000   $ 29,000,000,000 $ 0
Allowance for credit losses 4,000,000 4,000,000      
Pledged securities not separately reported $ 52,730,000,000 $ 52,730,000,000     $ 49,480,000,000
Percentage of investment portfolio considered investment grade 99.00% 99.00%      
Debt securities, held-to-maturity, allowance for credit loss $ 4,000,000 $ 4,000,000      
HTM securities writeoff $ 0        
HTM securities provision   4,000,000      
Losses from other-than-temporary impairment   1,000,000 $ 1,000,000    
Gross pre-tax unrealized losses   $ 210,000,000      
Number of securities in loss position | security   632