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Investment Securities - Investment Securities (Details) - USD ($)
$ in Millions
Jun. 30, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current $ 3,436.6 $ 3,101.8
Accumulated gross unrealized gain, current 14.7 23.8
Accumulated gross unrealized loss, current 128.8 103.5
Debt securities, current 3,322.5 3,022.1
Amortized cost, non-current 70.8 70.6
Accumulated gross unrealized gain, non-current 0.2 0.6
Accumulated gross unrealized loss, non-current 5.2 4.4
Fair value 65.7 66.8
Amortized Cost 3,507.4 3,172.4
Total Unrealized Gains 14.9 24.4
Total Unrealized Losses 134.0 107.9
Fair value 3,388.2 3,088.9
Executive deferred compensation plan trust 15.6 13.7
Asset Pledged as Collateral    
Debt Securities, Available-for-sale [Line Items]    
Fair value of debt securities pledged 82.5  
Asset Pledged as Collateral | FHLB advances | Federal Reserve Bank Advances    
Debt Securities, Available-for-sale [Line Items]    
Fair value of debt securities pledged 360.4  
Asset Pledged as Collateral | Borrowed federal funds | Federal Reserve Bank Advances    
Debt Securities, Available-for-sale [Line Items]    
Fair value of debt securities pledged 699.2  
Debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, non-current 28.5 28.6
Accumulated gross unrealized gain, non-current 0.2 0.6
Accumulated gross unrealized loss, non-current 1.3 0.8
Debt securities 27.4 28.4
U.S. treasury notes    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 429.9 410.1
Accumulated gross unrealized gain, current 0.1 0.8
Accumulated gross unrealized loss, current 35.0 32.3
Debt securities, current 395.1 378.6
Corporate and sovereign debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 1,227.5 1,086.8
Accumulated gross unrealized gain, current 5.5 13.6
Accumulated gross unrealized loss, current 35.3 31.6
Debt securities, current 1,197.6 1,068.8
Municipal bonds    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 70.9 70.8
Accumulated gross unrealized gain, current 0.2 0.3
Accumulated gross unrealized loss, current 5.8 5.4
Debt securities, current 65.4 65.7
Asset-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 668.0 582.6
Accumulated gross unrealized gain, current 5.9 3.2
Accumulated gross unrealized loss, current 2.7 4.2
Debt securities, current 671.2 581.6
Mortgage-backed securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 1,040.3 951.5
Accumulated gross unrealized gain, current 2.9 5.9
Accumulated gross unrealized loss, current 50.0 30.0
Debt securities, current 993.2 927.4
Mutual fund    
Debt Securities, Available-for-sale [Line Items]    
Equity securities FV-NI cost, non-current 29.4 29.1
Accumulated gross unrealized gain, non-current 0.0 0.0
Accumulated gross unrealized loss, non-current 3.9 3.6
Equity securities, FV-NI, noncurrent 25.4 25.5
Pooled investment fund    
Debt Securities, Available-for-sale [Line Items]    
Equity securities FV-NI cost, non-current 12.9 12.9
Accumulated gross unrealized gain, non-current 0.0 0.0
Accumulated gross unrealized loss, non-current 0.0 0.0
Equity securities, FV-NI, noncurrent $ 12.9 $ 12.9