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Loans and Allowance for Credit Losses - Additional Information (Detail)		3 Months Ended			9 Months Ended
	Jan. 31, 2025 USD ($)	Sep. 30, 2025 USD ($) Loan	Jun. 30, 2025 USD ($)	Sep. 30, 2024 USD ($) Loan	Sep. 30, 2025 USD ($) Loan	Sep. 30, 2024 USD ($) Loan	Dec. 31, 2024 USD ($)
Financing Receivable Allowance For Credit Losses [Line Items]
Loan portfolio purchases included in credit card receivables					$ 109,400,000
Proceeds from sales of loans held for sale					75,312,000	$ 60,779,000
Nonaccrual Loans		$ 131,965,000			131,965,000		$ 19,282,000
Restructured loans		177,000			177,000		196,000
Total Past Due		37,706,507,000			37,706,507,000		25,642,301,000
Non accrual loans with no related allowance for credit losses		62,453,000			62,453,000		19,282,000
Accrued interest on loans		178,200,000			$ 178,200,000		125,700,000
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	$ 8,006,000			$ 85,299,000
Heartland Financial USA Inc [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Fair value of loans acquired net of allowance for credit losses	$ 9,700,000,000
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					$ 75,300,000	$ 60,800,000
Residential Real Estate [Member]
Financing Receivable Allowance For Credit Losses [Line Items]
Number of Contracts \| Loan		1		0	3	2
Pre-modification loan balance		$ 401,000			$ 757,000	$ 291,000
Post-modification loan balance		432,000			$ 790,000	$ 293,000
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		$ 6,131,000			$ 6,131,000		$ 7,602,000