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Investment Securities - Pledged Securities and Concentration Narrative (Details) - USD ($)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2021
Dec. 31, 2020
Pledged securities      
Total pledged securities $ 3,600,000,000 $ 3,600,000,000 $ 2,400,000,000
Securities pledged to secure public deposits 3,377,544,000 3,377,544,000 2,251,508,000
Securities pledged to secure other financial transactions 193,100,000 193,100,000 186,100,000
Concentration of risk      
Credit losses recognized on securities 0 0 0
Non-government issuer      
Concentration of risk      
Securities of issuers in excess of 10% of stockholders' equity $ 0 $ 0 $ 0