| **Investment Securities - Investments in Debt and Equity Securities Reported Less than Historical Cost (Details) - USD ($) $ in Thousands** | Jun. 30, 2019 | Dec. 31, 2018 |
|---------------------------------------------------------------------------------------------------------------------------------------------|---------------|---------------|
| **Schedule of Investments [Line Items]**                                                                                                    |          |          |
| Fair value                                                                                                                                  | $ 156,285     | $ 932,577     |
| Investments reported at less than historical cost                                                                                           |          |          |
| **Schedule of Investments [Line Items]**                                                                                                    |          |          |
| Historical cost                                                                                                                             | 157,406       | 951,571       |
| Fair value                                                                                                                                  | 156,285       | 932,577       |
| Gross unrealized losses                                                                                                                     | $ 1,121       | $ 18,994      |
| Percent of the Corporation's investment portfolio                                                                                           | 7.50%         | 57.10%        |

| X                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Investment Historical Cost
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of Total Investment Portfolio, Fair Value
+ References
No definition available.
+ Details**Name:**frme_PercentageOfTotalInvestmentPortfolioFairValue**Namespace Prefix:**frme_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities Continuous Unrealized Loss Position, Accumulated Loss
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Continuous Unrealized Loss Position, Fair Value
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
+ Details**Name:**us-gaap_ScheduleOfInvestmentsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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