| **Accumulated Other Comprehensive Income (Loss) - Changes in Balances of Each Component (Details) (USD $)In Thousands, unless otherwise specified** | 3 Months Ended                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Accumulated Other Comprehensive Income [Roll Forward]**                                                                                           |                                                                                                                                                                                                         |   |
| AOCI - beginning balance                                                                                                                            | $ (1,630)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax                                                                                                                                             | $ (6,410)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
| Other comprehensive income before reclassifications                                                                                                 | 151us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax                                                                                                                                       | 5,548us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax |
| Amounts reclassified from accumulated other comprehensive income                                                                                    | (436)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax                                                                                                                    | (154)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax |
| Period change                                                                                                                                       | (285)us-gaap_OtherComprehensiveIncomeLossNetOfTax                                                                                                                                                            | 5,394us-gaap_OtherComprehensiveIncomeLossNetOfTax |
| AOCI - ending balance                                                                                                                               | (1,915)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax                                                                                                                                               | (1,016)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
| Unrealized Gains (Losses) on Securities Available for Sale                                                                                          |                                                                                                                                                                                                         |   |
| **Accumulated Other Comprehensive Income [Roll Forward]**                                                                                           |                                                                                                                                                                                                         |   |
| AOCI - beginning balance                                                                                                                            | 14,098us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember                                               | 1,566us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
| Other comprehensive income before reclassifications                                                                                                 | 980us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember                                      | 5,207us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
| Amounts reclassified from accumulated other comprehensive income                                                                                    | (666)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember                   | (378)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
| Period change                                                                                                                                       | 314us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember                                                             | 4,829us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
| AOCI - ending balance                                                                                                                               | 14,412us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember                                               | 6,395us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
| Unrealized Gains (Losses) on Securities Available for Sale for which a Portion of Other-Than-Temporary Impairment has been Recognized in Income     |                                                                                                                                                                                                         |   |
| **Accumulated Other Comprehensive Income [Roll Forward]**                                                                                           |                                                                                                                                                                                                         |   |
| AOCI - beginning balance                                                                                                                            |                                                                                                                                                                                                    | (1,847)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
| Other comprehensive income before reclassifications                                                                                                 |                                                                                                                                                                                                    | 1,164us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
| Amounts reclassified from accumulated other comprehensive income                                                                                    |                                                                                                                                                                                                    |    |
| Period change                                                                                                                                       | 0us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember                                                                  | 1,164us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
| AOCI - ending balance                                                                                                                               | 0us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember                                                       | (683)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
| Unrealized Gains (Losses) on Cash Flow Hedges                                                                                                       |                                                                                                                                                                                                         |   |
| **Accumulated Other Comprehensive Income [Roll Forward]**                                                                                           |                                                                                                                                                                                                         |   |
| AOCI - beginning balance                                                                                                                            | (2,182)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember                          | (501)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
| Other comprehensive income before reclassifications                                                                                                 | (829)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember                | (823)us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
| Amounts reclassified from accumulated other comprehensive income                                                                                    | 230us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember | 224us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
| Period change                                                                                                                                       | (599)us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember                                       | (599)us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
| AOCI - ending balance                                                                                                                               | (2,781)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember                          | (1,100)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember |
| Unrealized Gains (Losses) on Defined Benefit Plans                                                                                                  |                                                                                                                                                                                                         |   |
| **Accumulated Other Comprehensive Income [Roll Forward]**                                                                                           |                                                                                                                                                                                                         |   |
| AOCI - beginning balance                                                                                                                            | (13,546)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember                                               | (5,628)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
| Other comprehensive income before reclassifications                                                                                                 |                                                                                                                                                                                                    |    |
| Amounts reclassified from accumulated other comprehensive income                                                                                    |                                                                                                                                                                                                    |    |
| Period change                                                                                                                                       | 0us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember                                                                 | 0us-gaap_OtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |
| AOCI - ending balance                                                                                                                               | $ (13,546)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember                                             | $ (5,628)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax/ us-gaap_StatementEquityComponentsAxis= us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember |

| X                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**frme_AccumulatedOtherComprehensiveIncomeRollForward**Namespace Prefix:**frme_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3
+ Details**Name:**us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax and reclassification adjustments of other comprehensive income (loss).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669619-108580
+ Details**Name:**us-gaap_OtherComprehensiveIncomeLossNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
+ References
No definition available.
+ Details**Name:**us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**duration |
| X                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetGainLossFromDesignatedOrQualifyingCashFlowHedgesMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_StatementEquityComponentsAxis=us-gaap_AccumulatedOtherThanTemporaryImpairmentMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |