| **Restriction on Cash and Due From Banks (Details) $ in Thousands** | Dec. 31, 2016 USD ($) |
|---------------------------------------------------------------------|-----------------------|
| **Restricted Cash and Cash Equivalents Items [Line Items]**         |                  |
| Cash balance, Federal Home Loan Bank And Federal Reserve Bank       | $ 5,969               |
| Cash reserve with Federal Reserve Bank                              | 24,153                |
| Interest-bearing Deposits                                           |                  |
| **Restricted Cash and Cash Equivalents Items [Line Items]**         |                  |
| Cash amount that exceeded federally insured limits                  | $ 87,431              |

| 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                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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_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:** |