SECURITIES (Securities available for sale) (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Total debt securities	$ 1,139,138
Gross Unrealized Gains	2,798	$ 78,269
Gross Unrealized Losses	(682,269)	(84,233)
Estimated Fair Value	913,083
Equity securities with a readily determinable fair value	20,258	20,788
Securities carried at fair value, cost	6,739,802	7,536,338
Carried at fair value (cost of $6,739,802 at September 30, 2022, and $7,536,338 at December 31, 2021)	6,060,331	7,530,374
Total debt securities
Debt Securities, Available-for-sale [Line Items]
Total debt securities	6,719,544	7,515,550
Gross Unrealized Gains	2,798	78,269
Gross Unrealized Losses	(682,269)	(84,233)
Estimated Fair Value	6,040,073	7,509,586
U.S. treasuries
Debt Securities, Available-for-sale [Line Items]
Total debt securities	27,364	997
Gross Unrealized Gains	0	11
Gross Unrealized Losses	(627)	0
Estimated Fair Value	26,737	1,008
U.S. agencies
Debt Securities, Available-for-sale [Line Items]
Total debt securities	49,604	193,932
Gross Unrealized Gains	0	264
Gross Unrealized Losses	(6,304)	(812)
Estimated Fair Value	43,300	193,384
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total debt securities	1,052,960	2,045,386
Gross Unrealized Gains	1	56,263
Gross Unrealized Losses	(218,338)	(16,616)
Estimated Fair Value	834,623	2,085,033
Mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Total debt securities	2,097,286	2,388,601
Gross Unrealized Gains	67	11,870
Gross Unrealized Losses	(269,077)	(51,182)
Estimated Fair Value	1,828,276	2,349,289
Mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Total debt securities	2,261,005	1,749,838
Gross Unrealized Gains	2,730	4,570
Gross Unrealized Losses	(140,876)	(11,029)
Estimated Fair Value	2,122,859	1,743,379
Commercial mortgage-backed securities - agency
Debt Securities, Available-for-sale [Line Items]
Total debt securities	102,289	125,397
Gross Unrealized Gains	0	1,429
Gross Unrealized Losses	(16,141)	(2,914)
Estimated Fair Value	86,148	123,912
Commercial mortgage-backed securities - non-agency
Debt Securities, Available-for-sale [Line Items]
Total debt securities	708,763	600,253
Gross Unrealized Gains	0	998
Gross Unrealized Losses	(19,088)	(363)
Estimated Fair Value	689,675	600,888
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Total debt securities	411,063	408,167
Gross Unrealized Gains	0	2,803
Gross Unrealized Losses	(11,031)	(1,317)
Estimated Fair Value	400,032	409,653
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Total debt securities	9,210	2,979
Gross Unrealized Gains	0	61
Gross Unrealized Losses	(787)	0
Estimated Fair Value	$ 8,423	$ 3,040

Marketable Securities, Amortized Cost Basis

No definition available.

Name:	htlf_MarketableSecuritiesAmortizedCostBasis
Namespace Prefix:	htlf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

+ References

No definition available.

+ Details

Name:	us-gaap_EquitySecuritiesFvNiCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI https://asc.fasb.org/extlink&oid=126897435&loc=d3e534808-122878

+ Details

Name:	us-gaap_MarketableSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=htlf_CommercialMortgageBackedSecuritiesAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=htlf_CommercialMortgageBackedSecuritiesNonAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: