| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Details) - USD ($) $ in Thousands** | Dec. 31, 2016 | Dec. 31, 2015 |
|-------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | $ 5,095,918   | $ 4,641,215   |
| Loans 90 Days And Accruing                                                                                  | 112           | 907           |
| Non-Accrual                                                                                                 | 29,998        | 31,389        |
| Total Past Due & Non-Accrual                                                                            | 43,727        | 52,607        |
| Total                                                                                                       | 5,139,645     | 4,693,822     |
| 30-59 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 9,490         | 17,570        |
| 60-89 Days Past Due                                                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 4,127         | 2,741         |
| Commercial and industrial loans                                                                             |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 1,192,079     | 1,052,275     |
| Loans 90 Days And Accruing                                                                                  | 100           |               |
| Non-Accrual                                                                                                 | 1,839         | 4,634         |
| Total Past Due & Non-Accrual                                                                            | 2,567         | 4,800         |
| Total                                                                                                       | 1,194,646     | 1,057,075     |
| Commercial and industrial loans | 30-59 Days Past Due                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 466           | 166           |
| Commercial and industrial loans | 60-89 Days Past Due                                                       |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 162           |               |
| Agricultural production financing and other loans to farmers                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 78,360        | 96,884        |
| Loans 90 Days And Accruing                                                                                  |          |               |
| Non-Accrual                                                                                                 | 1,329         | 827           |
| Total Past Due & Non-Accrual                                                                            | 1,329         | 827           |
| Total                                                                                                       | 79,689        | 97,711        |
| Agricultural production financing and other loans to farmers | 30-59 Days Past Due                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |               |               |
| Agricultural production financing and other loans to farmers | 60-89 Days Past Due                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |          |               |
| Construction                                                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 415,975       | 362,084       |
| Non-Accrual                                                                                                 | 73            | 736           |
| Total Past Due & Non-Accrual                                                                            | 2,728         | 4,620         |
| Total                                                                                                       | 418,703       | 366,704       |
| Construction | 30-59 Days Past Due                                                                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 2,655         | 3,884         |
| Construction | 60-89 Days Past Due                                                                          |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |               |               |
| Commercial and farmland                                                                                     |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 1,932,896     | 1,786,092     |
| Loans 90 Days And Accruing                                                                                  |               |               |
| Non-Accrual                                                                                                 | 15,754        | 11,277        |
| Total Past Due & Non-Accrual                                                                            | 20,166        | 16,829        |
| Total                                                                                                       | 1,953,062     | 1,802,921     |
| Commercial and farmland | 30-59 Days Past Due                                                               |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 1,385         | 5,552         |
| Commercial and farmland | 60-89 Days Past Due                                                               |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 3,027         |               |
| Residential                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 725,338       | 765,634       |
| Loans 90 Days And Accruing                                                                                  | 9             | 502           |
| Non-Accrual                                                                                                 | 9,523         | 11,818        |
| Total Past Due & Non-Accrual                                                                            | 13,831        | 20,471        |
| Total                                                                                                       | 739,169       | 786,105       |
| Residential | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 3,664         | 6,090         |
| Residential | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 635           | 2,061         |
| Home equity                                                                                                 |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 415,969       | 344,344       |
| Loans 90 Days And Accruing                                                                                  | 3             | 324           |
| Non-Accrual                                                                                                 | 1,457         | 1,952         |
| Total Past Due & Non-Accrual                                                                            | 2,556         | 4,269         |
| Total                                                                                                       | 418,525       | 348,613       |
| Home equity | 30-59 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 850           | 1,433         |
| Home equity | 60-89 Days Past Due                                                                           |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 246           | 560           |
| Consumer                                                                                                    |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 76,929        | 73,990        |
| Loans 90 Days And Accruing                                                                                  |               | 81            |
| Non-Accrual                                                                                                 | 23            | 145           |
| Total Past Due & Non-Accrual                                                                            | 550           | 727           |
| Total                                                                                                       | 77,479        | 74,717        |
| Consumer | 30-59 Days Past Due                                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 470           | 445           |
| Consumer | 60-89 Days Past Due                                                                              |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    | 57            | 56            |
| Lease financing receivables, net of unearned income                                                         |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 311           | 588           |
| Non-Accrual                                                                                                 |          |               |
| Total Past Due & Non-Accrual                                                                            |          |               |
| Total                                                                                                       | 311           | 588           |
| Other commercial loans                                                                                      |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Current                                                                                                     | 258,061       | 159,324       |
| Non-Accrual                                                                                                 |               |               |
| Total Past Due & Non-Accrual                                                                            |               | 64            |
| Total                                                                                                       | 258,061       | 159,388       |
| Other commercial loans | 60-89 Days Past Due                                                                |          |          |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                                        |          |          |
| Past Due                                                                                                    |               | $ 64          |

| X                                                                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, More Than Ninety Days Past Due and Still Accruing
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableRecordedInvestmentMoreThanNinetyDaysPastDueAndStillAccruing**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                  |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing Receivable, Recorded Investment, Past Due And Non-Accrual
+ References
No definition available.
+ Details**Name:**frme_FinancingReceivableRecordedInvestmentPastDueAndNonAccrual**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are current.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentCurrent**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Recorded Investment -URI http://asc.fasb.org/extlink&oid=65880944Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5111-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Recorded Investment in financing receivables that are past due at the balance sheet date.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Recorded Investment -URI http://asc.fasb.org/extlink&oid=65880944Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**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_FinancingReceivableRecordedInvestmentPastDueLineItems**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                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables30To59DaysPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivablesPeriodPastDueAxis=us-gaap_FinancingReceivables60To89DaysPastDueMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndIndustrialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_FarmingAndAgricultureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_CommercialAndFarmLandLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialRealEstateMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerLoanMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_FinanceLeasesPortfolioSegmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                     |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=frme_OtherCommercialLoansMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |