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Investment Securities - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
USD ($)
Investments
Dec. 31, 2020
USD ($)
Investments
Schedule of Held-to-maturity Securities [Line Items]    
Investment securities classified as held to maturity $ 866.3 $ 51.6
Investment securities in an unrealized loss position (securities) | Investments 292 181
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss $ 0.0  
Municipal Debt Securities, at Carrying Value $ 60.9  
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share, Percent Debt Securities 42.00%  
Debt Securities, Available-for-sale, Amortized Cost $ 4,011.0 $ 3,932.7
Debt Securities, Available-for-sale, Realized Gain 0.0  
Debt Securities, Held-to-maturity, Sold, Realized Gain (Loss)   0.0
Debt Securities, Available-for-sale, Realized Loss 0.0  
State, county and municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Investment securities classified as held to maturity 74.1 46.6
Debt Securities, Available-for-sale, Amortized Cost 458.0 $ 462.1
813940 Political Organizations [Member]    
Schedule of Held-to-maturity Securities [Line Items]    
Municipal Debt Securities, at Carrying Value 25.6  
Residential Mortgage Backed Securities    
Schedule of Held-to-maturity Securities [Line Items]    
Debt Securities, Available-for-sale, Amortized Cost 235.8  
Callable Within One Year    
Schedule of Held-to-maturity Securities [Line Items]    
Amortized cost of investment securities callable after one year but within five years 265.1  
Fair value of investment securities callable after one year but within five years 258.6  
Callable structured notes amortized costs $ 0.0