| **Fair Values of Financial Instruments - Estimated Fair Values of Financial Instruments (Details) - USD ($) $ in Thousands** | Dec. 31, 2018 | Dec. 31, 2017 |
|------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Assets:**                                                                                                                  |          |          |
| Investment securities available for sale                                                                                     | $ 1,142,195   | $ 999,947     |
| Investment securities held to maturity                                                                                       | 489,217       | 568,208       |
| Loans held for sale                                                                                                          | 4,778         | 7,216         |
| Quoted Prices in Active Markets for Identical Assets (Level 1)                                                               |          |          |
| **Assets:**                                                                                                                  |          |          |
| Cash and cash equivalents                                                                                                    | 139,247       | 154,905       |
| Interest-bearing time deposits                                                                                               | 36,963        | 35,027        |
| Investment securities available for sale                                                                                     |               |               |
| **Liabilities:**                                                                                                             |          |          |
| Deposits                                                                                                                     | 6,267,879     | 5,741,019     |
| Significant Other Observable Inputs (Level 2)                                                                                |          |          |
| **Assets:**                                                                                                                  |          |          |
| Investment securities available for sale                                                                                     | 1,138,867     | 995,969       |
| Investment securities held to maturity                                                                                       | 481,377       | 556,305       |
| Loans held for sale                                                                                                          | 4,778         | 7,216         |
| Federal Home Loan Bank stock                                                                                                 | 24,588        | 23,825        |
| Interest rate swap asset                                                                                                     | 12,083        | 7,323         |
| Interest receivable                                                                                                          | 40,881        | 37,130        |
| **Liabilities:**                                                                                                             |          |          |
| Deposits                                                                                                                     | 1,464,129     | 1,406,526     |
| **Borrowings:**                                                                                                              |          |          |
| Federal funds purchased                                                                                                      | 104,000       | 144,038       |
| Securities sold under repurchase agreements                                                                                  | 113,437       | 136,562       |
| Federal Home Loan Bank advances                                                                                              | 318,728       | 361,085       |
| Subordinated debentures and term loans                                                                                       | 127,298       | 120,085       |
| Interest rate swap liability                                                                                                 | 12,636        | 8,688         |
| Interest payable                                                                                                             | 5,607         | 4,390         |
| Significant Unobservable Inputs (Level 3)                                                                                    |          |          |
| **Assets:**                                                                                                                  |          |          |
| Investment securities available for sale                                                                                     | 3,328         | 3,978         |
| Investment securities held to maturity                                                                                       | 7,840         | 11,903        |
| Loans                                                                                                                        | 7,004,193     | 6,534,877     |
| Carrying Amount                                                                                                              |          |          |
| **Assets:**                                                                                                                  |          |          |
| Cash and cash equivalents                                                                                                    | 139,247       | 154,905       |
| Interest-bearing time deposits                                                                                               | 36,963        | 35,027        |
| Investment securities available for sale                                                                                     | 1,142,195     | 999,947       |
| Investment securities held to maturity                                                                                       | 490,387       | 560,655       |
| Loans held for sale                                                                                                          | 4,778         | 7,216         |
| Loans                                                                                                                        | 7,143,915     | 6,676,167     |
| Federal Home Loan Bank stock                                                                                                 | 24,588        | 23,825        |
| Interest rate swap asset                                                                                                     | 12,083        | 7,323         |
| Interest receivable                                                                                                          | 40,881        | 37,130        |
| **Liabilities:**                                                                                                             |          |          |
| Deposits                                                                                                                     | 7,754,593     | 7,172,530     |
| **Borrowings:**                                                                                                              |          |          |
| Federal funds purchased                                                                                                      | 104,000       | 144,038       |
| Securities sold under repurchase agreements                                                                                  | 113,512       | 136,623       |
| Federal Home Loan Bank advances                                                                                              | 314,986       | 414,377       |
| Subordinated debentures and term loans                                                                                       | 138,463       | 139,349       |
| Interest rate swap liability                                                                                                 | 12,636        | 8,688         |
| Interest payable                                                                                                             | $ 5,607       | $ 4,390       |

| X                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Borrowed Funds [Abstract]
+ References
No definition available.
+ Details**Name:**frme_BorrowedFundsAbstract**Namespace Prefix:**frme_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest Bearing Deposits with Banks, Fair Value Disclosure
+ References
No definition available.
+ Details**Name:**frme_InterestBearingDepositsWithBanksFairValueDisclosure**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                               |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest Payable Fair Value Disclosure
+ References
No definition available.
+ Details**Name:**frme_InterestPayableFairValueDisclosure**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                     |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest Receivable, Fair Value Disclosure
+ References
No definition available.
+ Details**Name:**frme_InterestReceivableFairValueDisclosure**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563
+ Details**Name:**us-gaap_AvailableForSaleSecuritiesDebtSecurities**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.
+ References
No definition available.
+ Details**Name:**us-gaap_CashAndCashEquivalentsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of deposit liabilities held by the entity, including, but not limited to, foreign and domestic, interest and noninterest bearing, demand deposits, saving deposits, negotiable orders of withdrawal (NOW) and time deposits.
+ References
No definition available.
+ Details**Name:**us-gaap_DepositsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of short term borrowing where a bank borrows, at the federal funds rate, from another bank.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
+ Details**Name:**us-gaap_FederalFundsPurchasedFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of borrowing from Federal Home Loan Bank (FHLBank).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.13(3)(a)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878
+ Details**Name:**us-gaap_FederalHomeLoanBankBorrowingsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of investment in debt security measured at amortized cost (held-to-maturity).
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563
+ Details**Name:**us-gaap_HeldToMaturitySecuritiesFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets.
+ References
No definition available.
+ Details**Name:**us-gaap_InterestRateDerivativeAssetsAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value as of the balance sheet date of interest rate derivative liabilities, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as liabilities.
+ References
No definition available.
+ Details**Name:**us-gaap_InterestRateDerivativeLiabilitiesAtFairValue**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of investments in Federal Home Loan Bank (FHLBank) stock.
+ References
No definition available.
+ Details**Name:**us-gaap_InvestmentInFederalHomeLoanBankStockFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -URI http://asc.fasb.org/subtopic&trid=2196772
+ Details**Name:**us-gaap_LoansReceivableFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The face amount of loans that will be sold to other entities.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522
+ Details**Name:**us-gaap_LoansReceivableHeldForSaleAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of other liabilities.
+ References
No definition available.
+ Details**Name:**us-gaap_OtherLiabilitiesFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value portion of securities that an entity sells and agrees to repurchase at a specified date for a specified price.
+ References
Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13433-108611
+ Details**Name:**us-gaap_SecuritiesLoanedOrSoldUnderAgreementsToRepurchaseFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
| X                                                                                                                                                                                       |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |