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Loans and Allowance for Credit Losses - Additional Information (Detail)		3 Months Ended
	Jan. 31, 2025 USD ($)	Mar. 31, 2026 USD ($) Loan	Sep. 30, 2025 USD ($)	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($) Loan	Dec. 31, 2025 USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Proceeds from sales of loans held for sale		$ 27,232,000			$ 16,389,000
Nonaccrual Loans		151,250,000				$ 144,666,000
Restructured loans		163,000				169,000
Total Past Due		40,134,325,000				38,779,408,000
Non accrual loans with no related allowance for credit losses		76,877,000				76,789,000
Accrued interest on loans		$ 180,100,000				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
Loan modifications made to borrowers experiencing financial difficulty | Loan		0			0
PCD ACL at acquisition	$ 85,299,000				$ 62,126,000
Heartland Financial USA Inc [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
PCD ACL at acquisition		$ 62,100,000	$ 8,000,000	$ 15,200,000
Consumer Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Proceeds from sales of loans held for sale		$ 27,200,000			$ 16,400,000
Residential Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Number of Contracts | Loan		3			1
Pre-modification loan balance		$ 534,000			$ 225,000
Post-modification loan balance		$ 538,000			$ 225,000
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		$ 14,924,000				$ 18,403,000

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