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Loans and Allowance for Credit Losses - Additional Information (Detail)		3 Months Ended			6 Months Ended
	Jan. 31, 2025 USD ($)	Jun. 30, 2026 USD ($) Loan	Sep. 30, 2025 USD ($)	Jun. 30, 2025 USD ($) Loan	Jun. 30, 2026 USD ($) Loan	Jun. 30, 2025 USD ($) Loan	Dec. 31, 2025 USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Proceeds from sales of loans held for sale					$ 60,667,000	$ 41,668,000
Nonaccrual Loans		$ 127,526,000			127,526,000		$ 144,666,000
Restructured loans		157,000			157,000		169,000
Total Past Due		41,149,726,000			41,149,726,000		38,779,408,000
Non accrual loans with no related allowance for credit losses		84,178,000			84,178,000		76,789,000
Accrued interest on loans		174,100,000			$ 174,100,000		176,100,000
Credit card payments delinquency period beyond which payments considered significantly delinquent					60 days
Number of reasonable and supportable forecast period					1 year
Modifications made to borrowers experiencing financial difficulty commitment to lend		$ 0		$ 0	$ 0	$ 0
Loan modifications made to borrowers experiencing financial difficulty \| Loan		0		0	0	0
PCD ACL at acquisition	$ 85,299,000			$ 15,167,000		$ 77,293,000
Heartland Financial USA Inc [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
PCD ACL at acquisition			$ 8,000,000	$ 15,200,000	$ 62,100,000
Consumer Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Proceeds from sales of loans held for sale					$ 60,700,000	$ 41,700,000
Residential Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Number of Contracts \| Loan				1	3	2
Pre-modification loan balance				$ 131,000	$ 534,000	$ 356,000
Post-modification loan balance				$ 133,000	$ 538,000	$ 358,000
Minimum [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Number of reasonable and supportable forecast period					1 year
Maximum [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Number of reasonable and supportable forecast period					3 years
Greater than 90 days Past Due and Accruing [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Total Past Due		$ 13,715,000			$ 13,715,000		$ 18,403,000