| **Accounting For Certain Loans Acquired In a Purchase - Contractually Required Payments (Details) (USD $)In Thousands, unless otherwise specified** | Feb. 10, 2012                                                                                                                                         | Dec. 31, 2014                                                                                                                                                                                                                      | Dec. 31, 2013                                                                                                                                                                                                                      |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| **Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]**                                     |                                                                                                                                                  |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Contractually required payments receivable at acquisition date                                                                                      | $ 31,143us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Nonaccretable difference                                                                                                                            | 9,688us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference                                                                                    |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Accretable difference                                                                                                                               | 4,152us-gaap_CertainLoansAcquiredInTransferAccretableYield                                                                                            |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Basis in loans at acquisition date                                                                                                                  | 17,303us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue                               |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Merger with Community Bancshares, Inc.                                                                                                              |                                                                                                                                                  |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| **Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]**                                     |                                                                                                                                                  |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Contractually required payments receivable at acquisition date                                                                                      |                                                                                                                                                  | 26,032us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member |                                                                                                                                                                                                                               |
| Nonaccretable difference                                                                                                                            |                                                                                                                                                  | 3,498us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                  |                                                                                                                                                                                                                               |
| Expected cash flows at acquisition date                                                                                                             |                                                                                                                                                  | 22,534us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member          |                                                                                                                                                                                                                               |
| Accretable difference                                                                                                                               |                                                                                                                                                  | 2,234us-gaap_CertainLoansAcquiredInTransferAccretableYield/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                                                                                          |                                                                                                                                                                                                                               |
| Basis in loans at acquisition date                                                                                                                  |                                                                                                                                                  | 20,300us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCommunityBancsharesInc.Member                             |                                                                                                                                                                                                                               |
| Merger with CFS Bancorp, Inc. (Citizens)                                                                                                            |                                                                                                                                                  |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| **Certain Loans Acquired in Transfer Not Accounted for as Debt Securities Acquired During Period [Line Items]**                                     |                                                                                                                                                  |                                                                                                                                                                                                                               |                                                                                                                                                                                                                               |
| Contractually required payments receivable at acquisition date                                                                                      |                                                                                                                                                  |                                                                                                                                                                                                                               | 109,839us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodContractuallyRequiredPaymentsReceivableAtAcquisition/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember |
| Nonaccretable difference                                                                                                                            |                                                                                                                                                  |                                                                                                                                                                                                                               | 22,679us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                  |
| Expected cash flows at acquisition date                                                                                                             |                                                                                                                                                  |                                                                                                                                                                                                                               | 87,160us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember           |
| Accretable difference                                                                                                                               |                                                                                                                                                  |                                                                                                                                                                                                                               | 3,502us-gaap_CertainLoansAcquiredInTransferAccretableYield/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                                                                                           |
| Basis in loans at acquisition date                                                                                                                  |                                                                                                                                                  |                                                                                                                                                                                                                               | $ 83,658us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue/ us-gaap_BusinessAcquisitionAxis= frme_MergerwithCFSBancorpInc.CitizensMember                            |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
The aggregate excess of a loan's cash flows expected to be collected over the investor's initial investment in the loan. This relates to loans with deteriorating of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
No definition available.
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferAccretableYield**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the loan's contractually required payments receivable in excess of the amount of its cash flows expected to be collected. This relates to loans with deteriorating of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.
+ References
No definition available.
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferNonaccretableDifference**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of cash flows expected to be collected at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
+ Details**Name:**us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodCashFlowsExpectedToBeCollectedAtAcquisition**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of contractually required payments receivable at acquisition of acquired loans not accounted for as debt securities with evidence of deterioration of credit quality.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 15 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27045933&loc=d3e13503-111538Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544
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| X                                                                                                                                                                    |
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