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Investment Securities - Pledged Securities and Concentration Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Pledged securities    
Total pledged securities $ 2,600,000,000 $ 2,400,000,000
Securities pledged to secure public deposits 2,418,838,000 2,251,508,000
Securities pledged to secure other financial transactions 184,200,000 186,100,000
Concentration of risk    
Credit losses recognized on securities 0 0
Non-government issuer    
Concentration of risk    
Securities of issuers in excess of 10% of stockholders' equity $ 0 $ 0