| **Derivative Financial Instruments - Additional Information (Details) $ in Thousands**     | 9 Months Ended |          |
|--------------------------------------------------------------------------------------------|----------------|----------|
||
| **Derivative [Line Items]**                                                                |           |     |
| Estimated amount to be transferred from accumulated other comprehensive income to earnings | $ 1,130        |     |
| Termination value of derivatives in a net liability position                               | 16,813         |     |
| Derivative collateral posted                                                               | 19,021         |     |
| Not Designated as Hedging Instrument                                                       |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Notional amount of interest rate derivatives                                               | $ 240,285      |     |
| Federal Home Loan Bank Advances                                                            |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Number of debt instruments held | instrument                                               | 3              |     |
| Cash Flow Hedging | Interest Rate Swap                                                     |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Number of interest rate derivatives held | interest_rate_swap                              | 5              | 5        |
| Notional amount of interest rate derivatives                                               | $ 56,000       | $ 56,000 |
| Cash Flow Hedging | Interest Rate Swap | Trust Preferred Debt                              |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Notional amount of interest rate derivatives                                               | 26,000         |     |
| Cash Flow Hedging | Interest Rate Swap | Federal Home Loan Bank Advances                   |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Notional amount of interest rate derivatives                                               | $ 30,000       |     |
| Cash Flow Hedging | Interest Rate Cap                                                      |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Number of interest rate derivatives held | interest_rate_cap                               | 1              | 1        |
| Notional amount of interest rate derivatives                                               | $ 13,000       | $ 13,000 |
| Cash Flow Hedging | Interest Rate Cap | Trust Preferred Debt                               |           |     |
| **Derivative [Line Items]**                                                                |           |     |
| Notional amount of interest rate derivatives                                               | $ 13,000       |     |

| X                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number Of Debt Instruments Held
+ References
No definition available.
+ Details**Name:**frme_NumberOfDebtInstrumentsHeld**Namespace Prefix:**frme_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B
+ Details**Name:**invest_DerivativeNotionalAmount**Namespace Prefix:**invest_**Data Type:**xbrli:monetaryItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624258-113959
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The estimated value of gains (losses), net anticipated to be transferred in the future from accumulated other comprehensive income into earnings.
+ References
No definition available.
+ Details**Name:**us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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_DerivativeLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624258-113959
+ Details**Name:**us-gaap_DerivativeNetLiabilityPositionAggregateFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Number of interest rate derivative instruments held by the entity at the reporting date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5580258-113959Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 1A -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5579245-113959
+ Details**Name:**us-gaap_NumberOfInterestRateDerivativesHeld**Namespace Prefix:**us-gaap_**Data Type:**xbrli:integerItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                  |
|----------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
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