| **Accounting for Certain Loans Acquired in a Purchase - Narrative (Details) (USD $)In Millions, unless otherwise specified** | Feb. 10, 2012SCB Bank | Nov. 12, 2013Merger with CFS Bancorp, Inc. (Citizens) |
|------------------------------------------------------------------------------------------------------------------------------|-----------------------|-------------------------------------------------------|
| **Business Acquisition [Line Items]**                                                                                        |                  |                                                  |
| Loan Acquired during acquisition                                                                                             | $ 113.0               | $ 639.6                                               |
| Fair Value Discount                                                                                                          | $ 19.2                | $ 36.5                                                |

| X                                                                                                                                                                                                                                                                             |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Combination, Loans Acquired, Discount
+ References
No definition available.
+ Details**Name:**frme_BusinessCombinationLoansAcquiredDiscount**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Loans
+ References
No definition available.
+ Details**Name:**frme_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLoans**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BusinessAcquisitionLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |