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Securities - Additional Information (Detail) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Jan. 31, 2025
Dec. 31, 2024
Dec. 31, 2022
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Proceeds from sales of securities available for sale $ 7,871,000   $ 624,225,000 $ 19,154,000      
Fair Value 13,378,280,000   13,378,280,000     $ 7,774,334,000  
Gross realized gains from securities 91,000   514,000 139,000      
Accrued interest on securities available for sale 74,800,000   74,800,000     43,100,000  
ACL related to AFS Securities 0   0     0  
Sales of securities held to maturity 0 $ 0 0 0      
Amortized cost of available for sale securities transferred to held to maturity             $ 4,100,000,000
Fair value of available for sale securities transferred to held to maturity             $ 3,800,000,000
Gain (loss) on transfer of securities     0        
Unamortized unrealized loss 147,000,000   147,000,000     171,300,000  
Accrued interest on securities held to maturity 21,300,000   21,300,000     25,600,000  
Unrealized gain (loss) on trading securities     (6,000) $ 44,000      
Securities sold not yet purchased 13,000,000   13,000,000     7,100,000  
Securities held to maturity with an acquired fair value 5,119,701,000   5,119,701,000     4,748,938,000  
Other securities 719,575,000   719,575,000     471,018,000  
Heartland Financial USA Inc [Member]              
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Securities held to maturity with an acquired fair value         $ 438,900,000    
Other securities         105,300,000    
Available For Sale Security [Member] | Heartland Financial USA Inc [Member]              
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Fair Value         3,100,000,000    
FRB and FHLB stock [Member] | Heartland Financial USA Inc [Member]              
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Other securities         2,000,000    
Equity Securities Without Readily Determinable Fair Values [Member] | Heartland Financial USA Inc [Member]              
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Other securities         $ 103,200,000    
Collateral Pledged [Member] | U.S. Government and Other Public Deposit [Member]              
Schedule of Trading Securities and Other Trading Assets [Line Items]              
Pledged securities for deposits $ 11,600,000,000   $ 11,600,000,000     $ 10,500,000,000