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Other Comprehensive Income/Loss - Schedule of Accumulated Other Comprehensive Income (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Dec. 31, 2024
Jun. 30, 2025
Jun. 30, 2024
Dec. 31, 2023
Accumulated Other Comprehensive Income (Loss) [Line Items]                
Total stockholders’ equity $ 3,448.7 $ 3,365.8 $ 3,448.7 $ 3,365.8 $ 3,304.0 $ 3,421.8 $ 3,225.3 $ 3,227.5
Net unrealized gain (loss) on derivatives     0.2   (2.4)      
Change in net unrealized gain during period, pre-tax 38.1 158.5 153.5 134.6        
Change in net unrealized gain during period, tax expense (benefit) 9.5 39.4 38.3 33.5        
OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax 28.6 119.1 115.2 101.1        
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax 0.1 (0.2) 0.0 (0.5)        
Amortization of unrealized gains on available-for-sale securities transferred into held-to-maturity, tax expense (benefit) 0.0 0.0 0.0 0.0        
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax 0.1 (0.2) 0.0 (0.5)        
Unrealized loss on derivatives, tax expense (benefit) 0.1 0.8 0.5 2.9        
Other comprehensive income, before tax 38.4 173.8 157.0 135.2        
Other Comprehensive Income (Loss), tax expense (benefit) 9.6 43.2 39.2 33.6        
Other Comprehensive Income (Loss), Net of Tax 28.8 130.6 117.8 101.6        
Net unrealized loss on investment securities available-for-sale                
Accumulated Other Comprehensive Income (Loss) [Line Items]                
Total stockholders’ equity (198.4)   (198.4)   (313.6)      
OCI, Debt Securities, Available-for-Sale, Transfer from Held-to-Maturity, Gain (Loss), before Adjustment, after Tax     (5.9)   (5.9)      
Net accumulated other comprehensive loss                
Accumulated Other Comprehensive Income (Loss) [Line Items]                
Total stockholders’ equity $ (204.1) $ (254.9) $ (204.1) $ (254.9) $ (321.9) $ (232.9) $ (385.5) $ (356.5)