| **Restriction on Cash and Due From Banks (Details) (USD $)In Thousands, unless otherwise specified** | Dec. 31, 2014                                                                                                                                   |
|------------------------------------------------------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------------------------------------|
| **Restricted Cash and Cash Equivalents Items [Line Items]**                                          |                                                                                                                                            |
| Cash balance, Federal Home Loan Bank And Federal Reserve Bank                                        | $ 13,910frme_CashBalanceFederalHomeLoanBankAndFederalReserveBank                                                                                |
| Cash reserve with Federal Reserve Bank                                                               | 22,595frme_ReserveRequirementFederalReserveBank                                                                                                 |
| Interest-bearing Deposits                                                                            |                                                                                                                                            |
| **Restricted Cash and Cash Equivalents Items [Line Items]**                                          |                                                                                                                                            |
| Cash amount that exceeded federally insured limits                                                   | $ 103,154us-gaap_CashUninsuredAmount/ us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis= us-gaap_InterestBearingDepositsMember |

| 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
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 |
| X                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis=us-gaap_InterestBearingDepositsMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |