| **Loans and Allowance - Credit Quality of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|-------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | $ 5,139,645   | $ 4,693,822   |
| Commercial and industrial loans                                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,194,646     | 1,057,075     |
| Agricultural production financing and other loans to farmers                                                |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 79,689        | 97,711        |
| Construction                                                                                                |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 418,703       | 366,704       |
| Commercial and farmland                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,953,062     | 1,802,921     |
| Residential                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 739,169       | 786,105       |
| Home equity                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 418,525       | 348,613       |
| Consumer                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 77,479        | 74,717        |
| Finance Leases                                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 311           | 588           |
| Other commercial loans                                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 258,061       | 159,388       |
| Commercial Pass                                                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 3,769,920     | 3,404,491     |
| Commercial Pass | Commercial and industrial loans                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,117,545     | 962,340       |
| Commercial Pass | Agricultural production financing and other loans to farmers                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 30,712        | 77,884        |
| Commercial Pass | Construction                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 398,646       | 345,449       |
| Commercial Pass | Commercial and farmland                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,811,367     | 1,679,141     |
| Commercial Pass | Residential                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 146,251       | 171,576       |
| Commercial Pass | Home equity                                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 7,310         | 8,218         |
| Commercial Pass | Finance Leases                                                                            |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 228           | 495           |
| Commercial Pass | Other commercial loans                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 257,861       | 159,388       |
| Commercial Special Mention                                                                                  |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 124,863       | 105,965       |
| Commercial Special Mention | Commercial and industrial loans                                                |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 30,919        | 48,432        |
| Commercial Special Mention | Agricultural production financing and other loans to farmers                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 25,273        | 6,665         |
| Commercial Special Mention | Construction                                                                   |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 3,490         | 1,271         |
| Commercial Special Mention | Commercial and farmland                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 60,028        | 46,442        |
| Commercial Special Mention | Residential                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 5,106         | 3,107         |
| Commercial Special Mention | Home equity                                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 47            | 48            |
| Commercial Substandard                                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 159,215       | 149,395       |
| Commercial Substandard | Commercial and industrial loans                                                    |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 46,182        | 45,984        |
| Commercial Substandard | Agricultural production financing and other loans to farmers                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 23,704        | 13,162        |
| Commercial Substandard | Construction                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,858         | 1,790         |
| Commercial Substandard | Commercial and farmland                                                            |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 80,626        | 77,338        |
| Commercial Substandard | Residential                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 6,046         | 10,428        |
| Commercial Substandard | Home equity                                                                        |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 516           | 600           |
| Commercial Substandard | Finance Leases                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 83            | 93            |
| Commercial Substandard | Other commercial loans                                                             |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 200           |               |
| Commercial Doubtful                                                                                         |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |               | 319           |
| Commercial Doubtful | Commercial and industrial loans                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |               | 319           |
| Commercial Doubtful | Residential                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       |          |               |
| Consumer Performing                                                                                         |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,076,417     | 1,023,856     |
| Consumer Performing | Construction                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 14,636        | 18,114        |
| Consumer Performing | Commercial and farmland                                                               |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,034         |          |
| Consumer Performing | Residential                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 574,054       | 593,533       |
| Consumer Performing | Home equity                                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 409,237       | 337,718       |
| Consumer Performing | Consumer                                                                              |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 77,456        | 74,491        |
| Consumer Non Performing                                                                                     |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 9,230         | 9,796         |
| Consumer Non Performing | Construction                                                                      |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 73            | 80            |
| Consumer Non Performing | Commercial and farmland                                                           |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 7             |               |
| Consumer Non Performing | Residential                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 7,712         | 7,461         |
| Consumer Non Performing | Home equity                                                                       |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | 1,415         | 2,029         |
| Consumer Non Performing | Consumer                                                                          |          |          |
| **Financing Receivable, Recorded Investment [Line Items]**                                                  |          |          |
| Loans                                                                                                       | $ 23          | $ 226         |

| 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_FinancingReceivableRecordedInvestmentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=65884687&loc=d3e4428-111522
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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| X                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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