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Goodwill and Other Intangibles - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended
Sep. 02, 2025
Jan. 31, 2025
Mar. 31, 2026
Dec. 31, 2025
Dec. 31, 2024
Finite Lived Intangible Assets [Line Items]          
Goodwill     $ 1,837,594 $ 1,839,825 $ 207,385
Heartland Financial USA, Inc [Member]          
Finite Lived Intangible Assets [Line Items]          
Goodwill   $ 1,630,209      
Core deposit intangible asset   474,100      
Wealth customer list   26,000      
Purchased credit card relationships   $ 10,900      
Adjustment to goodwill     $ 2,200    
Healthcare Savings Account Business [Member]          
Finite Lived Intangible Assets [Line Items]          
Deposit acquired $ 32,500        
Core deposit intangible asset $ 4,800