Fair Values of Financial Instruments - Fair Value Measurements of Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	$ 1,142,195	$ 999,947
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	1,138,867	995,969
Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	3,328	3,978
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Interest rate swap liability
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liability	12,636	8,688
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. Government-sponsored agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	13,582
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	602,842	522,750
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. Government-sponsored mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	522,443	470,866
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities		2,353
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Interest rate swap asset
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset derivatives	11,948	7,305
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Interest rate cap
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset derivatives	135	18
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	3,293	3,943
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| U.S. Government-sponsored mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	4
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Corporate obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	31	31
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities		4
Fair Value, Measurements, Recurring \| Fair Value \| Interest rate swap liability
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swap liability	12,636	8,688
Fair Value, Measurements, Recurring \| Fair Value \| U.S. Government-sponsored agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	13,582
Fair Value, Measurements, Recurring \| Fair Value \| State and municipal
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	606,135	526,693
Fair Value, Measurements, Recurring \| Fair Value \| U.S. Government-sponsored mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	522,447	470,866
Fair Value, Measurements, Recurring \| Fair Value \| Corporate obligations
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investment securities available for sale	31	31
Fair Value, Measurements, Recurring \| Fair Value \| Equity securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Equity securities		2,357
Fair Value, Measurements, Recurring \| Fair Value \| Interest rate swap asset
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset derivatives	11,948	7,305
Fair Value, Measurements, Recurring \| Fair Value \| Interest rate cap
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Asset derivatives	$ 135	$ 18

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

+ Details

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

- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109247956&loc=SL51823488-111719

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

+ Details

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

- Definition

Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=99377092&loc=SL75136599-209740

+ Details

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

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=frme_InterestRateSwapLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_FairValueByAssetClassAxis=frme_InterestRateSwapAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_InterestRateCapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: