v3.25.2
Accumulated Other Comprehensive Income (Loss) - Summary of Changes in Accumulated Other Comprehensive Income (Detail) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance $ 7,214.5 $ 6,172.5 $ 6,707.5 $ 6,078.4
Net current-period other comprehensive loss (3.1) (0.6) 52.1 (45.3)
Ending balance 7,406.8 6,334.2 7,406.8 6,334.2
Unrealized holding gains (losses) on AFS securities        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance (480.6) (560.8) (534.7) (516.6)
Other comprehensive income (loss) before reclassifications 9.7 1.7 65.4 (43.2)
Amounts reclassified from AOCI (8.8) (1.8) (10.4) (1.1)
Net current-period other comprehensive loss 0.9 (0.1) 55.0 (44.3)
Ending balance (479.7) (560.9) (479.7) (560.9)
Unrealized holding gains (losses) on junior subordinated debt        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance 2.5 2.3 1.4 2.8
Other comprehensive income (loss) before reclassifications (4.1) (0.5) (3.0) (1.0)
Amounts reclassified from AOCI 0.0 0.0 0.0 0.0
Net current-period other comprehensive loss (4.1) (0.5) (3.0) (1.0)
Ending balance (1.6) 1.8 (1.6) 1.8
Impairment loss on securities        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance 0.0 1.2 0.0 1.2
Other comprehensive income (loss) before reclassifications 0.0 0.0 0.0 0.0
Amounts reclassified from AOCI 0.0 0.0 0.0 0.0
Net current-period other comprehensive loss 0.0 0.0 0.0 0.0
Ending balance 0.0 1.2 0.0 1.2
Accumulated Other Comprehensive Income (Loss)        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance (478.5) (557.6) (533.7) (512.9)
Other comprehensive income (loss) before reclassifications 5.7 1.2 62.5 (44.2)
Amounts reclassified from AOCI (8.8) (1.8) (10.4) (1.1)
Net current-period other comprehensive loss (3.1) (0.6) 52.1 (45.3)
Ending balance (481.6) (558.2) (481.6) (558.2)
Unrealized holding gains (losses) on SERP        
Accumulated Other Comprehensive Income (Loss) [Line Items]        
Beginning balance (0.4) (0.3) (0.4) (0.3)
Other comprehensive income (loss) before reclassifications 0.1 0.0 0.1 0.0
Amounts reclassified from AOCI 0.0 0.0 0.0 0.0
Net current-period other comprehensive loss 0.1 0.0 0.1 0.0
Ending balance $ (0.3) $ (0.3) $ (0.3) $ (0.3)