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Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Change in net unrealized loss during period $ (113.7) $ 61.8 $ 54.9  
Reclassification adjustment for net gains included in net income (1.1) (0.3) (0.1)  
Accumulated other comprehensive income (loss) 1,986.6 1,959.8 $ 2,013.9 $ 1,693.9
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), after Adjustments, before Tax, Noncontrolling Interest 4.2 0.2    
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), Adjustments, Tax 1.1 0.0    
Other Comprehensive Income (Loss), Derivative, Excluded Component, Increase (Decrease), after Adjustments, Tax 3.1 0.2    
Net unrealized (loss) gain on investment securities available-for-sale        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Accumulated other comprehensive income (loss) $ (29.0) $ 56.8