INVESTMENT SECURITIES - Narrative (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021	Jan. 01, 2021
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Accrued interest on investment securities	$ 27,500	$ 26,800
Allowance for credit losses	245	245
Carrying value of securities pledged as collateral	965,800	873,200
Securities sold under repurchase agreements	231,800	175,100
State and municipal
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Allowance for credit losses	$ 245	$ 245
State and municipal \| Cumulative effect of ASC 326 adoption
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Allowance for credit losses			$ 245

Debt Securities, Available-for-sale and Held-to-maturity, Accrued Interest

No definition available.

Name:	frme_DebtSecuritiesAvailableForSaleAndHeldToMaturityAccruedInterest
Namespace Prefix:	frme_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Schedule Of Available For Sale Securities And Held To Maturity Securities

+ References

No definition available.

+ Details

Name:	frme_ScheduleOfAvailableForSaleSecuritiesAndHeldToMaturitySecuritiesLineItems
Namespace Prefix:	frme_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Securities Pledged as Collateral

+ References

No definition available.

+ Details

Name:	frme_SecuritiesPledgedAsCollateral
Namespace Prefix:	frme_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, excluding accrued interest, of allowance for credit loss on investment in debt security measured at amortized cost (held-to-maturity).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

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

- Definition

Amount of asset associated with funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 20
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226038-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI https://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: