v3.26.1
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, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance $ 7,908.1 $ 7,214.5 $ 7,945.9 $ 6,707.5
Net current-period other comprehensive income (loss) 4.3 (3.1) (107.0) 52.1
Ending balance 8,135.3 7,406.8 8,135.3 7,406.8
Unrealized holding gains (losses) on AFS securities        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance (452.7) (480.6) (340.7) (534.7)
Other comprehensive income (loss) before reclassifications 5.9 9.7 (68.3) 65.4
Amounts reclassified from AOCI (0.7) (8.8) (38.5) (10.4)
Net current-period other comprehensive income (loss) 5.2 0.9 (106.8) 55.0
Ending balance (447.5) (479.7) (447.5) (479.7)
Unrealized holding gains (losses) on junior subordinated debt        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance (2.5) 2.5 (3.5) 1.4
Other comprehensive income (loss) before reclassifications (0.9) (4.1) 0.1 (3.0)
Amounts reclassified from AOCI 0.0 0.0 0.0 0.0
Net current-period other comprehensive income (loss) (0.9) (4.1) 0.1 (3.0)
Ending balance (3.4) (1.6) (3.4) (1.6)
Accumulated Other Comprehensive Income (Loss)        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance (455.5) (478.5) (344.2) (533.7)
Other comprehensive income (loss) before reclassifications 5.0 5.7 (68.5) 62.5
Amounts reclassified from AOCI (0.7) (8.8) (38.5) (10.4)
Net current-period other comprehensive income (loss) 4.3 (3.1) (107.0) 52.1
Ending balance (451.2) (481.6) (451.2) (481.6)
Unrealized holding gains (losses) on SERP | Unrealized holding gains (losses) on SERP        
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]        
Beginning balance (0.3) (0.4) 0.0 (0.4)
Other comprehensive income (loss) before reclassifications 0.0 0.1 (0.3) 0.1
Amounts reclassified from AOCI 0.0 0.0 0.0 0.0
Net current-period other comprehensive income (loss) 0.0 0.1 (0.3) 0.1
Ending balance $ (0.3) $ (0.3) $ (0.3) $ (0.3)