| **Disclosures About Fair Value of Assets and Liabilities (Tables)**                                                                                                                    | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Fair Value Disclosures [Abstract]**                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Fair value measurements of assets and liabilities recognized in Consolidated Condensed Balance Sheets measured at fair value                                                           | The following table presents the fair value measurements of assets and liabilities recognized in the Consolidated Condensed Balance Sheets measured at fair value on a recurring basis and the level within the fair value hierarchy in which the fair value measurements fall at March 31, 2016, and December 31, 2015.  Fair Value Measurements Using:March 31, 2016Fair ValueQuoted Prices in Active Markets for Identical Assets(Level 1)Significant OtherObservable Inputs(Level 2)SignificantUnobservableInputs(Level 3)Available for sale securities:       U.S. Government-sponsored agency securities$103   $103  State and municipal330,120   324,651 $5,469U.S. Government-sponsored mortgage-backed securities302,586   302,586  Corporate obligations31     31Equity securities3,945   3,941 4Interest rate swap asset8,635   8,635  Interest rate cap12   12  Interest rate swap liability13,656   13,656    Fair Value Measurements Using:December 31, 2015Fair ValueQuoted Prices inActive Markets forIdentical Assets(Level 1)Significant Other Observable Inputs(Level 2)SignificantUnobservableInputs(Level 3)Available for sale securities:    U.S. Government-sponsored agency securities$104 $104 State and municipal305,911 300,014$5,897U.S. Government-sponsored mortgage-backed securities346,266 346,266 Certificates of deposit2,176   2,176  Corporate obligations31  31Equity securities3,912 3,9084Interest rate swap asset4,938 4,938Interest rate cap36 36Interest rate swap liability8,070 8,070                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
| Reconciliation of beginning and ending balances of recurring fair value measurements recognized in Consolidated Condensed Balance Sheets using significant unobservable Level 3 inputs | The following is a reconciliation of the beginning and ending balances of recurring fair value measurements recognized in the Consolidated Condensed Balance Sheets using significant unobservable (Level 3) inputs for three months ended March 31, 2016 and 2015.  Available for Sale Securities Three Months EndedMarch 31, 2016Three Months Ended March 31, 2015Balance at beginning of the period$5,932$6,646Total realized and unrealized gains and losses: Included in other comprehensive income3891Principal payments(466)(539)Ending balance $5,504$6,198                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Description of valuation methodologies used for instruments measured at fair value on a non-recurring basis and recognized in Consolidated Condensed Balance Sheets                    | The following table presents the fair value measurement of assets and liabilities measured at fair value on a nonrecurring basis and the level within the fair value hierarchy in which the fair value measurements fall at March 31, 2016, and December 31, 2015.  Fair Value Measurements UsingMarch 31, 2016Fair ValueQuoted Prices inActive Markets forIdentical Assets(Level 1)Significant OtherObservableInputs(Level 2)Significant UnobservableInputs(Level 3)Impaired loans (collateral dependent)$9,530  $9,530Other real estate owned1,287  1,287    Fair Value Measurements UsingDecember 31, 2015Fair ValueQuoted Prices inActive Markets forIdentical Assets(Level 1)Significant OtherObservable Inputs(Level 2)Significant UnobservableInputs(Level 3)Impaired loans (collateral dependent)$7,066  $7,066Other real estate owned5,529  5,529                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Unobservable inputs used in recurring and nonrecurring Level 3 fair value measurements other than goodwill                                                                             | The following table presents quantitative information about unobservable inputs used in recurring and nonrecurring Level 3 fair value measurements, other than goodwill, at March 31, 2016 and December 31, 2015.March 31, 2016Fair ValueValuation TechniqueUnobservable InputsRange                  (Weighted-Average)State and municipal securities$5,469Discounted cash flowMaturity/Call date1 month to 15 yrs   Blend of US Muni BQ curveA- to BBB-   Discount rate.69% - 5%        Corporate obligations and Equity securities$35Discounted cash flowRisk free rate3 month LIBOR   plus premium for illiquidityplus 200bps        Impaired loans (collateral dependent)$9,530Collateral based measurementsDiscount to reflect current market conditions and ultimate collectability0% - 50%                                        (1%)   Other real estate owned$1,287AppraisalsDiscount to reflect current market conditions0% - 20%                               (2%)December 31, 2015Fair Value Valuation Technique Unobservable Inputs Range                  (Weighted-Average)State and municipal securities$5,897 Discounted cash flow Maturity/Call date 1 month to 15 yrs     Blend of US Muni BQ curve A- to BBB-     Discount rate .80% - 5%        Corporate obligations and Equity securities$35 Discounted cash flow Risk free rate 3 month LIBOR     plus premium for illiquidity plus 200bps        Impaired loans (collateral dependent)$7,066 Collateral based measurements Discount to reflect current market conditions and ultimate collectability 0% - 50%                                        (2%)        Other real estate owned$5,529 Appraisals Discount to reflect current market conditions 0% - 20%                               (2%)                                                                                                                                                                                                                                                                                                                                                       |
| Estimated fair values of financial instruments                                                                                                                                         | The following table presents estimated fair values of the Corporation’s financial instruments and the level within the fair value hierarchy in which the fair value measurements fall at March 31, 2016, and December 31, 2015.March 31, 2016 CarryingAmountQuoted Prices in Active Marketsfor IdenticalAssetsSignificantOtherObservableInputsSignificant UnobservableInputs (Level 1)(Level 2)(Level 3)Assets:    Cash and cash equivalents$85,738$85,738  Interest-bearing time deposits77,45377,453  Investment securities available for sale636,785 $631,281$5,504Investment securities held to maturity633,992 626,16429,281Loans held for sale3,628 3,628 Loans4,647,821  4,600,417Federal Reserve Bank and Federal Home Loan Bank stock37,553 37,553 Interest rate swap and cap asset8,647 8,647 Interest receivable22,836 22,836 Liabilities:    Deposits$5,311,070$4,139,907$1,158,592 Borrowings:  Federal funds purchased602   602  Securities sold under repurchase agreements150,134 150,118 Federal Home Loan Bank advances263,588 265,170 Subordinated debentures and term loans127,523 98,807 Interest rate swap liability13,656 13,656 Interest payable3,910 3,910 December 31, 2015 CarryingAmountQuoted Prices in Active Marketsfor IdenticalAssetsSignificantOtherObservableInputsSignificant UnobservableInputs (Level 1)(Level 2)(Level 3)Assets:    Cash and cash equivalents$102,170$102,170  Interest-bearing time deposits32,31532,315  Investment securities available for sale658,400 $652,468$5,932Investment securities held to maturity618,599 598,08234,298Loans held for sale9,894 9,894 Loans4,631,369  4,539,940Federal Reserve Bank and Federal Home Loan Bank stock37,633 37,633 Interest rate swap and cap asset4,974 4,974Interest receivable24,415 24,415 Liabilities:    Deposits$5,289,647$4,095,004$1,177,142 Borrowings:    Federal funds purchased49,72149,721Securities sold under repurchase agreements155,325 155,325 Federal Home Loan Bank advances235,652 236,375 Subordinated debentures and term loans127,846 103,643 Interest rate swap liability8,070 8,070Interest payable3,092 3,092  |

| X                                                                                                                                                                                                                                                                                                                                                                                         |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair Value, Assets And Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation
+ References
No definition available.
+ Details**Name:**frme_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTableTextBlock**Namespace Prefix:**frme_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the inputs and valuation techniques used to measure fair value, and a discussion of changes in valuation techniques and related inputs, if any, applied during the period to each separate class of assets, liabilities, and financial instruments classified in shareholders' equity that are measured on a recurring and/or nonrecurring basis.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of assets and liabilities by class, including financial instruments measured at fair value that are classified in shareholders' equity, if any, that are measured at fair value on a nonrecurring basis in periods after initial recognition (for example, impaired assets). Disclosures may include, but are not limited to: (a) the fair value measurements recorded and the reasons for the measurements and (b) the level within the fair value hierarchy in which the fair value measurements are categorized in their entirety (levels 1, 2, 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19296-110258
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnNonrecurringBasisTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13476-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13467-108611Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6447952&loc=d3e13220-108610Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611
+ Details**Name:**us-gaap_FairValueByBalanceSheetGroupingTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FairValueDisclosuresAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19190-110258
+ Details**Name:**us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |