| **Acquisitions and Divestitures (Tables)**                                                      | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
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| Ameriana                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| **Business Acquisition [Line Items]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Schedule of preliminary valuations of the fair value of assets acquired and liabilities assumed | Based on preliminary valuations of the fair value of tangible and intangible assets acquired and liabilities assumed, which are based on assumptions that are subject to change, the purchase price for the Ameriana acquisition is detailed in the following table.  If prior to the end of the one year measurement period for finalizing the purchase price allocation, information becomes available which would indicate adjustments are required to the purchase price allocation, such adjustments will be included in the purchase price allocation retrospectively.  Fair ValueCash and cash equivalents $4,068Interest-bearing time deposits 8,790Investment securities 61,754Loans 316,929Premises and equipment 13,946Federal Home Loan Bank stock 2,693Other real estate owned 5,613Interest receivable 1,306Cash surrender value of life insurance 28,188Other assets 6,713Deposits (383,718)Interest payable (24)Federal Home Loan Bank Advances (24,884)Subordinated debentures (5,487)Other liabilities (9,451)Net tangible assets acquired 26,436Core deposit intangible 5,342Goodwill 38,624Purchase price $70,402 |
| C Financial                                                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| **Business Acquisition [Line Items]**                                                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Schedule of preliminary valuations of the fair value of assets acquired and liabilities assumed | The purchase price of the C Financial acquisition was allocated as follows:  Fair ValueCash and cash equivalents $2,496Federal Funds sold 7,018Interest-bearing time deposits 922Loans 110,625Premises and equipment 7,290Federal Home Loan Bank stock 855Interest receivable 292Other assets 119Deposits (105,326)Interest payable (29)Federal Home Loan Bank Advances(18,958)Other liabilities (2,911)Net tangible assets acquired 2,393Core deposit intangible 981Goodwill 11,126Purchase price $14,500                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |

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| - 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.
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| - Definition
Tabular disclosure of the amounts recognized as of the acquisition date for each major class of assets acquired and liabilities assumed. May include but not limited to the following: (a) acquired receivables; (b) contingencies recognized at the acquisition date; and (c) the fair value of noncontrolling interests in the acquiree.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=73530397&loc=d3e4845-128472
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| - Details**Name:**us-gaap_BusinessAcquisitionAxis=frme_AmerianaBancorpMember**Namespace Prefix:****Data Type:**na**Balance Type:****Period Type:** |
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