| **Business Combination - Pro Forma Information (Details) (USD $)In Thousands, except Per Share data, unless otherwise specified** | 12 Months Ended                                                                  |
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| **Business Acquisition [Line Items]**                                                                                             |                                                                             |   |   |
| Total revenue (net interest income plus other income)                                                                             | $ 263,070frme_BusinessAcquisitionProFormaRevenueNetInterestIncomePlusOtherIncome | $ 253,668frme_BusinessAcquisitionProFormaRevenueNetInterestIncomePlusOtherIncome | $ 266,034frme_BusinessAcquisitionProFormaRevenueNetInterestIncomePlusOtherIncome |
| Net income                                                                                                                        | 61,572frme_BusinessAcquisitionProFormaNetIncomeLossAttributabletoParent          | 39,979frme_BusinessAcquisitionProFormaNetIncomeLossAttributabletoParent | 50,092frme_BusinessAcquisitionProFormaNetIncomeLossAttributabletoParent |
| Net income available to common shareholders                                                                                       | $ 61,572us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss                        | $ 37,599us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss | $ 45,553us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss |
| **Earnings per share:**                                                                                                           |                                                                             |   |   |
| Basic                                                                                                                             | $ 1.63us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic                   | $ 0.98us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic | $ 1.28us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic |
| Diluted                                                                                                                           | $ 1.61us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted                 | $ 0.97us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted | $ 1.27us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted |

| X                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Pro Forma Net Income Loss, Attributable to Parent
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                        |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Business Acquisition, Pro Forma Revenue, Net Interest Income Plus Other Income
+ References
No definition available.
+ Details**Name:**frme_BusinessAcquisitionProFormaRevenueNetInterestIncomePlusOtherIncome**Namespace Prefix:**frme_**Data Type:**xbrli:monetaryItemType**Balance Type:**credit**Period Type:**duration |
| X                                                                                                                                                                |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The pro forma basic net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The pro forma diluted net income per share for a period as if the business combination or combinations had been completed at the beginning of a period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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| X                                                                                                                                                                                  |
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The pro forma net Income or Loss for the period as if the business combination or combinations had been completed at the beginning of a period.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)-(3) -URI http://asc.fasb.org/extlink&oid=25497992&loc=d3e1392-128463
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