| **Borrowings - Schedule of Securities Sold Under Repurchase Agreements (Details) (Securities Sold Under Repurchase Agreements, USD $)In Thousands, unless otherwise specified** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Securities Sold Under Repurchase Agreements                                                                                                                                     |            |   |
| **Debt Instrument [Line Items]**                                                                                                                                                |            |   |
| Maximum Amount Outstanding During Period                                                                                                                                        | $ 161,814       | $ 160,127 |
| Average Outstanding Amount                                                                                                                                                      | $ 140,126       | $ 144,555 |

| X                                                                                                                                                                                                                                                                      |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Average Outstanding Amount
+ References
No definition available.
+ Details**Name:**frme_LongTermDebtAverageOutstandingAmount**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Long-term Debt, Maximum Amount Outstanding During Period
+ References
No definition available.
+ Details**Name:**frme_LongTermDebtMaximumAmountOutstandingDuringPeriod**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_DebtInstrumentLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |