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Goodwill and Other Intangibles - Changes in Carrying Amount of Goodwill (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2026
Dec. 31, 2025
Goodwill And Other Intangible Assets [Line Items]    
Goodwill $ 1,839,825 $ 207,385
Acquisition of HTLF (2,231) 1,632,440
Goodwill 1,837,594 1,839,825
Commercial Banking [Member]    
Goodwill And Other Intangible Assets [Line Items]    
Goodwill 1,042,577 63,113
Acquisition of HTLF (1,339) 979,464
Goodwill 1,041,238 1,042,577
Institutional Banking [Member]    
Goodwill And Other Intangible Assets [Line Items]    
Goodwill 76,492 76,492
Goodwill 76,492 76,492
Personal Banking [Member]    
Goodwill And Other Intangible Assets [Line Items]    
Goodwill 720,756 67,780
Acquisition of HTLF (892) 652,976
Goodwill $ 719,864 $ 720,756