| **Loans and Allowance - Past Due Aging of Loan Portfolio by Loan Class (Detail) (USD $)** | Dec. 31, 2012   | Dec. 31, 2011   |
|-------------------------------------------------------------------------------------------|-----------------|-----------------|
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | $ 2,828,813,000 | $ 2,605,479,000 |
| 30-59 Days Past Due                                                                       | 14,303,000      | 25,835,000      |
| 60-89 Days Past Due                                                                       | 3,657,000       | 11,929,000      |
| Loans > 90 Days And Accruing                                                           | 2,037,000       | 580,000         |
| Non- Accrual                                                                              | 53,399,000      | 69,592,000      |
| Total Past Due & Non- Accrual                                                         | 73,396,000      | 107,936,000     |
| Total Loans                                                                               | 2,902,209,000   | 2,713,415,000   |
| Commercial and industrial                                                                 |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 607,442,000     | 518,764,000     |
| 30-59 Days Past Due                                                                       | 2,628,000       | 1,332,000       |
| 60-89 Days Past Due                                                                       | 144,000         | 135,000         |
| Loans > 90 Days And Accruing                                                           | 170,000         | 46,000          |
| Non- Accrual                                                                              | 12,195,000      | 12,246,000      |
| Total Past Due & Non- Accrual                                                         | 15,137,000      | 13,759,000      |
| Total Loans                                                                               | 622,579,000     | 532,523,000     |
| Agriculture production financing and other loans                                          |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 112,527,000     | 104,464,000     |
| 30-59 Days Past Due                                                                       |            | 62,000          |
| Total Past Due & Non- Accrual                                                         |            | 62,000          |
| Total Loans                                                                               | 112,527,000     | 104,526,000     |
| Construction                                                                              |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 93,426,000      | 69,305,000      |
| 30-59 Days Past Due                                                                       | 399,000         | 328,000         |
| 60-89 Days Past Due                                                                       |            | 3,126,000       |
| Loans > 90 Days And Accruing                                                           |            | 31,000          |
| Non- Accrual                                                                              | 4,814,000       | 8,990,000       |
| Total Past Due & Non- Accrual                                                         | 5,213,000       | 12,475,000      |
| Total Loans                                                                               | 98,639,000      | 81,780,000      |
| Commercial and farm land                                                                  |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 1,238,907,000   | 1,140,897,000   |
| 30-59 Days Past Due                                                                       | 3,276,000       | 16,457,000      |
| 60-89 Days Past Due                                                                       | 1,822,000       | 5,783,000       |
| Loans > 90 Days And Accruing                                                           | 65,000          |            |
| Non- Accrual                                                                              | 22,612,000      | 31,093,000      |
| Total Past Due & Non- Accrual                                                         | 27,775,000      | 53,333,000      |
| Total Loans                                                                               | 1,266,682,000   | 1,194,230,000   |
| Residential                                                                               |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 453,743,000     | 458,925,000     |
| 30-59 Days Past Due                                                                       | 5,734,000       | 5,485,000       |
| 60-89 Days Past Due                                                                       | 1,338,000       | 2,087,000       |
| Loans > 90 Days And Accruing                                                           | 1,246,000       | 191,000         |
| Non- Accrual                                                                              | 11,476,000      | 14,805,000      |
| Total Past Due & Non- Accrual                                                         | 19,794,000      | 22,568,000      |
| Total Loans                                                                               | 473,537,000     | 481,493,000     |
| Home Equity                                                                               |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 199,063,000     | 187,788,000     |
| 30-59 Days Past Due                                                                       | 1,467,000       | 1,096,000       |
| 60-89 Days Past Due                                                                       | 323,000         | 590,000         |
| Loans > 90 Days And Accruing                                                           | 556,000         | 261,000         |
| Non- Accrual                                                                              | 1,997,000       | 1,896,000       |
| Total Past Due & Non- Accrual                                                         | 4,343,000       | 3,843,000       |
| Total Loans                                                                               | 203,406,000     | 191,631,000     |
| Consumer                                                                                  |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 74,919,000      | 82,837,000      |
| 30-59 Days Past Due                                                                       | 799,000         | 1,075,000       |
| 60-89 Days Past Due                                                                       | 30,000          | 208,000         |
| Loans > 90 Days And Accruing                                                           |            | 51,000          |
| Non- Accrual                                                                              |            | 1,000           |
| Total Past Due & Non- Accrual                                                         | 829,000         | 1,335,000       |
| Total Loans                                                                               | 75,748,000      | 84,172,000      |
| Finance Leases                                                                            |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 2,289,000       | 3,555,000       |
| Non- Accrual                                                                              | 301,000         |       |
| Total Past Due & Non- Accrual                                                         | 301,000         |            |
| Total Loans                                                                               | 2,590,000       | 3,555,000       |
| Other Loans                                                                               |            |            |
| **Financing Receivable, Recorded Investment, Past Due [Line Items]**                      |            |            |
| Current                                                                                   | 46,497,000      | 38,944,000      |
| Non- Accrual                                                                              | 4,000           | 561,000         |
| Total Past Due & Non- Accrual                                                         | 4,000           | 561,000         |
| Total Loans                                                                               | $ 46,501,000    | $ 39,505,000    |

| 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 receivables that are less than 60 days past due but more than 29 days past due.
+ 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=7512638&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestment30To59DaysPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Financing receivables that are less than 90 days past due but more than 59 days past due.
+ 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=7512638&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestment60To89DaysPastDue**Namespace Prefix:**us-gaap_**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=7512638&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 -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=7512638&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 -Topic 310 -SubTopic 10 -Section 50 -Paragraph 7A -URI http://asc.fasb.org/extlink&oid=7512638&loc=SL6953401-111524
+ Details**Name:**us-gaap_FinancingReceivableRecordedInvestmentPastDue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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=6375948&loc=d3e4428-111522Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 01-6 -Paragraph 13 -Subparagraph e -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_LoansAndLeasesReceivableGrossCarryingAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |