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Securities - Summary of Portfolio of Securities Available for Sale (Parenthetical) (Details) - USD ($)
$ in Millions
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Available-for-sale Securities      
Schedule of Available-for-sale Securities [Line Items]      
Accrued interest receivable $ 14.8 $ 14.9 $ 24.4