| **Document and Entity Information Document** | Jun. 30, 2020                               |
|----------------------------------------------|---------------------------------------------|
| **Cover [Abstract]**                         |                                        |
| Document Type                                | 8-K                                         |
| Document Period End Date                     | Jun. 30,  2020                              |
| Entity File Number                           | 0-17071                                     |
| Entity Registrant Name                       | FIRST MERCHANTS CORP                        |
| Entity Incorporation, State or Country Code  | IN                                          |
| Entity Tax Identification Number             | 35-1544218                                  |
| Entity Address, Address Line One             | 200 East Jackson Street                     |
| Entity Address, Address Line Two             | P.O. Box 792                                |
| Entity Address, City or Town                 | Muncie                                      |
| Entity Address, State or Province            | IN                                          |
| Entity Address, Postal Zip Code              | 47305-2814                                  |
| City Area Code                               | 765                                         |
| Local Phone Number                           | 747-1500                                    |
| Title of 12(b) Security                      | Common Stock, $0.125 stated value per share |
| Trading Symbol                               | FRME                                        |
| Security Exchange Name                       | NASDAQ                                      |
| Written Communications                       | false                                       |
| Soliciting Material                          | false                                       |
| Pre-commencement Tender Offer                | false                                       |
| Pre-commencement Issuer Tender Offer         | false                                       |
| Entity Emerging Growth Company               | false                                       |
| Entity Central Index Key                     | 0000712534                                  |
| Amendment Flag                               | false                                       |

| X                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
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Boolean flag that is true when the XBRL content amends previously-filed or accepted submission.
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| - Definition
Area code of city
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| - Definition
Cover page.
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| - Definition
The end date of the period reflected on the cover page if a periodic report. For all other reports and registration statements containing historical data, it is the date up through which that historical data is presented.  If there is no historical data in the report, use the filing date. The format of the date is CCYY-MM-DD.
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| X                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The type of document being provided (such as 10-K, 10-Q, 485BPOS, etc). The document type is limited to the same value as the supporting SEC submission type, or the word 'Other'.
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No definition available.
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| X                                                                                                                                                                                                                                                                          |
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| - Definition
Address Line 1 such as Attn, Building Name, Street Name
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Address Line 2 such as Street or Suite number
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| X                                                                                                                                                                                                                                         |
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| - Definition
Name of the City or Town
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| - Definition
Code for the postal or zip code
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| - Definition
Name of the state or province.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
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| - Definition
A unique 10-digit SEC-issued value to identify entities that have filed disclosures with the SEC. It is commonly abbreviated as CIK.
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| - Definition
Indicate if registrant meets the emerging growth company criteria.
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| - Definition
Commission file number. The field allows up to 17 characters. The prefix may contain 1-3 digits, the sequence number may contain 1-8 digits, the optional suffix may contain 1-4 characters, and the fields are separated with a hyphen.
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| X                                                                                                                                                                                                                                                                                                        |
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| - Definition
Two-character EDGAR code representing the state or country of incorporation.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| - Definition
The exact name of the entity filing the report as specified in its charter, which is required by forms filed with the SEC.
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| - Definition
The Tax Identification Number (TIN), also known as an Employer Identification Number (EIN), is a unique 9-digit value assigned by the IRS.
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| - Definition
Local phone number for entity.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act.
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Title of a 12(b) registered security.
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Name of the Exchange on which a security is registered.
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| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as soliciting material pursuant to Rule 14a-12 under the Exchange Act.
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Trading symbol of an instrument as listed on an exchange.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Boolean flag that is true when the Form 8-K filing is intended to satisfy the filing obligation of the registrant as written communications pursuant to Rule 425 under the Securities Act.
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