| **Accounting for Certain Loans Acquired in a Purchase (Tables)**                                                 | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Receivables [Abstract]**                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Schedule of outstanding balance and carrying amount of loans acquired during the period                          | The following table includes the outstanding balance and carrying amount of loans acquired during the years ended December 31, 2012, 2013 and 2014, which are included in the balance sheet amounts of loans receivable at March 31, 2015 and December 31, 2014.March 31, 2015December 31, 2014CommunityCFSSCBTotalCommunityCFSSCBTotalCommercial and industrial loans$6,828$58,252$5,673$70,753$8,168$64,897$6,059$79,124Agricultural production financing and other loans to farmers7378771,6141,1008931,993Real estate loans:  Construction19,9697,33427,30319,0639,11328,176Commercial and farmland69,421232,43614,666316,52374,600251,00215,593341,195Residential28,048137,4576,868172,37328,863144,3967,384180,643       Home Equity9,32437,43914,84361,6069,88139,24415,75864,883Individuals' loans for household and other personal expenditures1,056779951,9301,3149221212,357Other Loans84848686Total$135,383$473,781$43,022$652,186$142,989$509,660$45,808$698,457Carrying Amount$126,996$450,867$36,980$614,843$134,198$484,949$39,324$658,471Allowance195285480650650Carrying Amount Net of Allowance$126,801$450,582$36,980$614,363$134,198$484,299$39,324$657,821 |
| Schedule of accretable yield, or income expected to be collected, and reclassifications from nonaccretable yield | The accretable yield, or income expected to be collected, and reclassifications from nonaccretable yield, are identified in the table below.  The table reflects only purchased loans accounted for under ASC 310-30 and not the entire portfolio of purchased loans.Three Months Ended March 31, 2015 Three Months Ended March 31, 2014CommunityCFSSCBTotalCFSSCBTotalBeginning balance$2,122$2,400$868$5,390$4,164$1,388$5,552AdditionsAccretion(179)(1,341)(185)(1,705)(301)(187)(488)Reclassification from nonaccretable479501351,13225255307Disposals(35)(35)Ending balance$1,990$2,009$818$4,817$4,080$1,256$5,336                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |

| X                                                                                                                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Schedule of Carrying Values of Certain Loans Acquired in Transfer Not Accounted for as Debt Securities [Table Text Block]
+ References
No definition available.
+ Details**Name:**frme_ScheduleOfCarryingValuesOfCertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesTableTextBlock**Namespace Prefix:**frme_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Schedule of Certain Loans Acquired in Transfer Not Accounted for as Debt Securities, Accretable Yield Movement [Table Text Block]
+ References
No definition available.
+ Details**Name:**frme_ScheduleOfCertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYieldMovementTableTextBlock**Namespace Prefix:**frme_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_ReceivablesAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |