| **Share-Based Compensation - Additional Information (Details) - USD ($)**            | 3 Months Ended | 9 Months Ended | 12 Months Ended        | 21 Months Ended |
|--------------------------------------------------------------------------------------|----------------|----------------|------------------------|-----------------|
||
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Percentage of average closing price to be paid by employees                          |           |           | 85.00%                 |            |   |   |
| Maximum common stock purchases through advance payroll deductions in a calendar year |           |           | $ 25,000               |            |   |   |
| Share-based compensation                                                             | $ 648,000      | $ 556,000      | $ 1,883,000            | $ 1,687,000     | $ 1,687,000 |   |
| Options granted (in shares)                                                          |           |           | 0                      | 0               |   | 0 |
| Forfeiture rate                                                                      |           |           | 3.80%                  |            |   |   |
| Unrecognized compensation expense related to stock options                           | $ 0            |           | $ 0                    |            |   | $ 0 |
| Aggregate intrinsic value of stock options exercised                                 |           |           | 245,000                | $ 812,000       |   |   |
| Cash receipts of stock options exercised                                             |           |           | $ 309,000              | $ 1,422,000     |   |   |
| Stock Options                                                                        |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Stock option term                                                                    |           |           | 10 years               |            |   |   |
| Stock options vesting percentage                                                     |           |           | 100.00%                |            |   |   |
| Stock Options | Minimum                                                              |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Vested period                                                                        |           |           | 6 months               |            |   |   |
| Stock Options | Maximum                                                              |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Vested period                                                                        |           |           | 2 years                |            |   |   |
| RSAs                                                                                 |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Vested period                                                                        |           |           | 3 years                |            |   |   |
| Outstanding DSUs (in shares)                                                         | 357,576        |           | 357,576                |            | 354,504 | 357,576 |
| Unrecognized compensation expense related to RSAs                                    | $ 4,129,000    |           | $ 4,129,000            |            |   | $ 4,129,000 |
| Unrecognized compensation expense expected recognition period                        |           |           | 1 year 6 months 2 days |            |   |   |
| DSUs | Non-employee Director                                                         |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Outstanding DSUs (in shares)                                                         | 0              |           | 0                      |            |   | 0 |
| ESPP                                                                                 |           |           |                   |            |   |   |
| **Share-based Compensation Arrangement by Share-based Payment Award [Line Items]**   |           |           |                   |            |   |   |
| Unrecognized compensation expense related to stock options                           | $ 0            |           | $ 0                    |            |   | $ 0 |
| Grant date fair value of ESPP options                                                | $ 18,000       |           |                   |            |   |   |

| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The maximum amount allowed for advance payroll deductions in a calendar year for employee stock purchase plan.
+ References
No definition available.
+ Details**Name:**frme_EmployeeStockPurchasePlanEmployeeContributionAmount**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share Based Compensation Arrangement by Share Based Payment Award Contractual Term Range
+ References
No definition available.
+ Details**Name:**frme_ShareBasedCompensationArrangementByShareBasedPaymentAwardContractualTermRange**Namespace Prefix:**frme_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share based Compensation Arrangement by Share based Payment Award, Fair Value Assumptions, Annual Forfeiture Rate
+ References
No definition available.
+ Details**Name:**frme_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAnnualForfeitureRate**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share Based Compensation Arrangement By Share Based Payment Award Grant Date Fair Value Of Awards
+ References
No definition available.
+ Details**Name:**frme_ShareBasedCompensationArrangementByShareBasedPaymentAwardGrantDateFairValueOfAwards**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share Based Compensation Arrangement by Share based Payment Award, Vesting Percentage
+ References
No definition available.
+ Details**Name:**frme_ShareBasedCompensationArrangementByShareBasedPaymentAwardVestingPercentage**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Share Based Compensation Arrangements By Share Based Payment Award Percentage Of Market Price At Eligible Employees Granted Options To Purchase Shares
+ References
No definition available.
+ Details**Name:**frme_ShareBasedCompensationArrangementsByShareBasedPaymentAwardPercentageOfMarketPriceAtEligibleEmployeesGrantedOptionsToPurchaseShares**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
+ Details**Name:**us-gaap_AllocatedShareBasedCompensationExpense**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
+ Details**Name:**us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unrecognized cost of unvested share-based awards, other than options, awarded to employees as compensation.
+ References
No definition available.
+ Details**Name:**us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Unrecognized cost of unvested options awarded to employees as compensation.
+ References
No definition available.
+ Details**Name:**us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The cash inflow associated with the amount received from holders exercising their stock options. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3255-108585Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228
+ Details**Name:**us-gaap_ProceedsFromStockOptionsExercised**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
+ Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1**Namespace Prefix:**us-gaap_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
+ Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**instant |
| 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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901
+ Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Gross number of share options (or share units) granted during the period.
+ References
No definition available.
+ Details**Name:**us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross**Namespace Prefix:**us-gaap_**Data Type:**xbrli:sharesItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MinimumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RangeAxis=us-gaap_MaximumMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                           |
|---------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AwardTypeAxis=frme_DeferredStockUnitsDSUsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_TitleOfIndividualAxis=frme_NonemployeeDirectorMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_AwardTypeAxis=us-gaap_EmployeeStockMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |