| **Fair Values of Financial Instruments - Narrative (Detail) (USD $)**                            | 12 Months Ended                                                                                                                                                                         |
|--------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]** |                                                                                                                                                                                    |   |   |   |
| Gross gains on securities sold                                                                   | $ 3,581,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains                                                                                                                         | $ 487,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains | $ 2,389,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains |   |
| Investment securities available for sale                                                         | 549,543,000us-gaap_AvailableForSaleSecurities                                                                                                                                           | 536,201,000us-gaap_AvailableForSaleSecurities |   |   |
| Significant Unobservable Inputs (Level 3)                                                        |                                                                                                                                                                                    |   |   |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]** |                                                                                                                                                                                    |   |   |   |
| Investment securities available for sale                                                         | 6,646,000us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member                                                        | 9,977,000us-gaap_AvailableForSaleSecurities/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |   |   |
| Significant Unobservable Inputs (Level 3) | Available for Sale Securities                        |                                                                                                                                                                                    |   |   |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]** |                                                                                                                                                                                    |   |   |   |
| Transfer between Levels 1, 2 and 3                                                               |                                                                                                                                                                                    | 350,000us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet/ us-gaap_FairValueByAssetClassAxis= us-gaap_AvailableforsaleSecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |   |   |
| Significant Unobservable Inputs (Level 3) | Held to Maturity Securities                          |                                                                                                                                                                                    |   |   |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]** |                                                                                                                                                                                    |   |   |   |
| Transfer between Levels 1, 2 and 3                                                               |                                                                                                                                                                                    | (16,784,000)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet/ us-gaap_FairValueByAssetClassAxis= us-gaap_HeldtomaturitySecuritiesMember/ us-gaap_FairValueByFairValueHierarchyLevelAxis= us-gaap_FairValueInputsLevel3Member |   |   |
| Corporate obligations                                                                            |                                                                                                                                                                                    |   |   |   |
| **Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]** |                                                                                                                                                                                    |   |   |   |
| Gross gains on securities sold                                                                   | 1,900,000us-gaap_AvailableForSaleSecuritiesGrossRealizedGains/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember                                   |   |   |   |
| Accumulated OTTI on securities                                                                   | 9,400,000frme_SecuritiesWithOtherThanTemporaryImpairmentAccumulatedOtherThanTemporaryImpairment/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |   |   |   |
| Net impairment losses recognized in earnings                                                     |                                                                                                                                                                                    |   |   | $ 500,000us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNetAvailableforsaleSecurities/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis= us-gaap_CorporateDebtSecuritiesMember |

| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities With Other-Than-Temporary Impairment, Accumulated Other-Than-Temporary Impairment
+ References
No definition available.
+ Details**Name:**frme_SecuritiesWithOtherThanTemporaryImpairmentAccumulatedOtherThanTemporaryImpairment**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesGrossRealizedGains**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.
+ References
No definition available.
+ Details**Name:**us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersNet**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of other than temporary impairment losses on investments in debt and equity securities categorized as Available-for-sale which have been recognized in earnings during the period. Such impairment losses consist of other than temporary impairments (OTTI) on equity securities, credit losses on investments in debt securities, and when the entity intends to sell an impaired debt security or it is more likely than not that the entity will be required to sell the impaired debt security before recovery of its amortized cost basis.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563
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| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_CorporateDebtSecuritiesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |