| **Document and Entity Information Document**                                                                             | Oct. 25, 2022                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------|
| **Entity Listings [Line Items]**                                                                                         |                                                                                                                     |
| Document Type                                                                                                            | 8-K                                                                                                                      |
| Document Period End Date                                                                                                 | Oct. 25,  2022                                                                                                           |
| Entity Registrant Name                                                                                                   | FIRST MERCHANTS CORP                                                                                                     |
| Entity Incorporation, State or Country Code                                                                              | IN                                                                                                                       |
| Entity File Number                                                                                                       | 001-41342                                                                                                                |
| 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                                                                                                                 |
| 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                                                                                                                    |
| Common Stock, $0.125 stated value per share                                                                              |                                                                                                                     |
| **Entity Listings [Line Items]**                                                                                         |                                                                                                                     |
| Title of 12(b) Security                                                                                                  | Common Stock, $0.125 stated value per share                                                                              |
| Trading Symbol                                                                                                           | FRME                                                                                                                     |
| Security Exchange Name                                                                                                   | NASDAQ                                                                                                                   |
| Depositary Shares, each representing a 1/100th interest in a share of Non-Cumulative Perpetual Preferred Stock, Series A |                                                                                                                     |
| **Entity Listings [Line Items]**                                                                                         |                                                                                                                     |
| Title of 12(b) Security                                                                                                  | Depositary Shares, each representing a 1/100th interest in a share of Non-Cumulative Perpetual Preferred Stock, Series A |
| Trading Symbol                                                                                                           | FRMEP                                                                                                                    |
| Security Exchange Name                                                                                                   | NASDAQ                                                                                                                   |

| 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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
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| - Definition
For the EDGAR submission types of Form 8-K: the date of the report, the date of the earliest event reported; for the EDGAR submission types of Form N-1A: the filing date; for all other submission types: the end of the reporting or transition period.  The format of the date is YYYY-MM-DD.
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| X                                                                                                                                                                                                                                                                                                                                                                                    |
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| - 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'.
+ References
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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| - Definition
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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No definition available.
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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.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                          |
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| - Definition
Indicate if registrant meets the emerging growth company criteria.
+ References
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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.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                        |
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| - Definition
Two-character EDGAR code representing the state or country of incorporation.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                       |
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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.
+ Details**Name:**dei_EntityListingsLineItems**Namespace Prefix:**dei_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| 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.
+ References
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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.
+ References
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| X                                                                                                                                                                                                                                        |
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| - Definition
Local phone number for entity.
+ References
No definition available.
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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.
+ References
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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.
+ References
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| - Definition
Title of a 12(b) registered security.
+ References
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| - Definition
Name of the Exchange on which a security is registered.
+ References
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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.
+ References
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| X                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Trading symbol of an instrument as listed on an exchange.
+ References
No definition available.
+ Details**Name:**dei_TradingSymbol**Namespace Prefix:**dei_**Data Type:**dei:tradingSymbolItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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.
+ References
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| X                                                                                                                                                   |
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