| **Restriction on Cash and Due From Banks (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2012 |
|------------------------------------------------------------------------------------------------------|---------------|
| **Restricted Cash and Cash Equivalents Items [Line Items]**                                          |          |
| Non-interest Bearing Deposits At Other Institutions                                                  | $ 6,909       |
| Cash Balance, Federal Home Loan Bank And Federal Reserve Bank                                        | 3,508         |
| Cash Reserve With Federal Reserve Bank                                                               | 24,181        |
| Interest-bearing Deposits [Member]                                                                   |          |
| **Restricted Cash and Cash Equivalents Items [Line Items]**                                          |          |
| Cash Amount That Exceeded Federally Insured Limits                                                   | $ 33,228      |

| X                                                                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash Balance, Federal Home Loan Bank And Federal Reserve Bank
+ References
No definition available.
+ Details**Name:**frme_CashBalanceFederalHomeLoanBankAndFederalReserveBank**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Non-Interest Deposits In Banks and Other FInancial Institutions
+ References
No definition available.
+ Details**Name:**frme_NonInterestDepositsInBanksAndOtherFinancialInstitutions**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Reserve Requirement, Federal Reserve Bank
+ References
No definition available.
+ Details**Name:**frme_ReserveRequirementFederalReserveBank**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash as of the balance sheet date that is not insured by the Federal Deposit Insurance Corporation.
+ References
No definition available.
+ Details**Name:**us-gaap_CashUninsuredAmount**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                  |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RestrictedCashAndCashEquivalentsItemsLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |