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Gain (loss) on transfer of securities.

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Amount of investments classified as other.

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Securities - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
Securities - Additional Information (Detail) - USD ($)	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024	Jan. 31, 2025	Dec. 31, 2022
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 611,423,000	$ 19,153,000
Gross realized gains from securities	390,000	139,000
Fair Value	10,895,659,000		$ 7,774,334,000
Accrued interest on securities available for sale	58,000,000.0		43,100,000
ACL related to AFS Securities	0		0
Sales of securities held to maturity	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	163,000,000.0		171,300,000
Accrued interest on securities held to maturity	22,100,000		25,600,000
Unrealized gain (loss) on trading securities	15,000	$ 40,000
Securities sold not yet purchased	11,900,000		7,100,000
Securities held to maturity with an acquired fair value	5,107,059,000		4,748,938,000
Other securities	647,152,000		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				123,800,000
Available for Sale Securities [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]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other securities				2,000,000
Equity Securities Without Readily Determinable Fair Values [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other securities				$ 121,800,000
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	$ 11,200,000,000		$ 10,500,000,000