| **Fair Value of Financial Instruments - Unobservable Inputs Used in Recurring and Nonrecurring Level 3 Fair Value Measurements Other Than Goodwill (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|-----------------|
||
| Discounted Cash Flow | State and municipal securities                                                                                                                                                         |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Fair Value                                                                                                                                                                                                    | 18,100          |
| Discounted Cash Flow | State and municipal securities | Minimum                                                                                                                                               |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Maturity/Call date                                                                                                                                                                                            | 1 month         |
| Blend of US Muni BQ curve                                                                                                                                                                                     | A-              |
| Discount rate                                                                                                                                                                                                 | 1.00%           |
| Discounted Cash Flow | State and municipal securities | Maximum                                                                                                                                               |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Maturity/Call date                                                                                                                                                                                            | 11 years        |
| Blend of US Muni BQ curve                                                                                                                                                                                     | BBB-            |
| Discount rate                                                                                                                                                                                                 | 4.00%           |
| Discounted Cash Flow | Corporate Bonds and Notes                                                                                                                                                              |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Fair Value                                                                                                                                                                                                    | 228             |
| Discounted Cash Flow | 3 month libor | Corporate Bonds and Notes                                                                                                                                              |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Premium for illiquidity                                                                                                                                                                                       | 2.00%           |
| Collateral Based Measurements | Impaired loans (collateral dependent)                                                                                                                                         |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Fair Value                                                                                                                                                                                                    | 17,703          |
| Collateral Based Measurements | Impaired loans (collateral dependent) | Minimum                                                                                                                               |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                     | 0.00%           |
| Collateral Based Measurements | Impaired loans (collateral dependent) | Maximum                                                                                                                               |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Discount to reflect current market conditions and ultimate collectability                                                                                                                                     | 50.00%          |
| Appraisals | Other real estate owned                                                                                                                                                                          |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Fair Value                                                                                                                                                                                                    | 7,684           |
| Appraisals | Other real estate owned | Minimum                                                                                                                                                                |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Discount to reflect current market conditions                                                                                                                                                                 | 0.00%           |
| Appraisals | Other real estate owned | Maximum                                                                                                                                                                |            |
| **Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]**                                                                                                                    |            |
| Discount to reflect current market conditions                                                                                                                                                                 | 20.00%          |

| X                                                                                                                                                                                                                         |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities, Maturity
+ References
No definition available.
+ Details**Name:**frme_SecuritiesMaturity**Namespace Prefix:**frme_**Data Type:**xbrli:durationItemType**Balance Type:**na**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This element represents the aggregate of the assets reported on the balance sheet at period end measured at fair value by the entity. This element is intended to be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_AssetsFairValueDisclosure**Namespace Prefix:**us-gaap_**Data Type:**xbrli:monetaryItemType**Balance Type:**debit**Period Type:**instant |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Description of the credit rating indicative of likelihood of repayment of the debt instrument given by a credit rating agency (for example, Standard and Poor's, Moody's, Fitch).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 129 -Paragraph 2, 4 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.
+ Details**Name:**us-gaap_DebtInstrumentCreditRating**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Risk-free interest rate assumption used in valuing an instrument.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueAssumptionsRiskFreeInterestRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Adjustment to the discount rate used to measure fair value for lack of ability to convert business interests into cash quickly.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsDiscountForLackOfMarketability**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value.  For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsDiscountRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsPrepaymentRate**Namespace Prefix:**us-gaap_**Data Type:**num:percentItemType**Balance Type:**na**Period Type:**duration |