| **Document and Entity Information (USD $)In Thousands, except Share data, unless otherwise specified** | 12 Months Ended       |            |           |
|--------------------------------------------------------------------------------------------------------|-----------------------|------------|-----------|
||
| **Document Information [Line Items]**                                                                  |                  |       |      |
| Document Type                                                                                          | 10-K                  |       |      |
| Amendment Flag                                                                                         | false                 |       |      |
| Document Period End Date                                                                               | Dec. 31,         2012 |       |      |
| Document Fiscal Year Focus                                                                             | 2012                  |       |      |
| Document Fiscal Period Focus                                                                           | FY                    |       |      |
| Trading Symbol                                                                                         | FRME                  |       |      |
| Entity Registrant Name                                                                                 | FIRST MERCHANTS CORP  |       |      |
| Entity Central Index Key                                                                               | 0000712534            |       |      |
| Current Fiscal Year End Date                                                                           | --12-31               |       |      |
| Entity Filer Category                                                                                  | Accelerated Filer     |       |      |
| Entity Common Stock, Shares Outstanding                                                                |                  | 28,695,887 |      |
| Entity Well-known Seasoned Issuer                                                                      | No                    |       |      |
| Entity Current Reporting Status                                                                        | Yes                   |       |      |
| Entity Voluntary Filers                                                                                | No                    |       |      |
| Entity Public Float                                                                                    |                  |       | $ 356,895 |

| X                                                                                                                                                                                                                                                                                            |
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| X                                                                                                                                                                                                                                                                 |
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End date of current fiscal year in the format --MM-DD.
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| - Definition
This is focus fiscal period of the document report. For a first quarter 2006 quarterly report, which may also provide financial information from prior periods, the first fiscal quarter should be given as the fiscal period focus. Values: FY, Q1, Q2, Q3, Q4, H1, H2, M9, T1, T2, T3, M8, CY.
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This is focus fiscal year of the document report in CCYY format. For a 2006 annual report, which may also provide financial information from prior periods, fiscal 2006 should be given as the fiscal year focus. Example: 2006.
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| X                                                                                                                                                        |
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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                                                                                                                                                                                                                                                                                                                                                                                    |
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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".
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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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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
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| - Definition
Indicate number of shares or other units outstanding of each of registrant's classes of capital or common stock or other ownership interests, if and as stated on cover of related periodic report. Where multiple classes or units exist define each class/interest by adding class of stock items such as Common Class A [Member], Common Class B [Member] or Partnership Interest [Member] onto the Instrument [Domain] of the Entity Listings, Instrument.
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| - Definition
Indicate "Yes" or "No" whether registrants (1) have filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that registrants were required to file such reports), and (2) have been subject to such filing requirements for the past 90 days. This information should be based on the registrant's current or most recent filing containing the related disclosure.
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| - Definition
Indicate whether the registrant is one of the following: (1) Large Accelerated Filer, (2) Accelerated Filer, (3) Non-accelerated Filer, (4) Smaller Reporting Company (Non-accelerated) or (5) Smaller Reporting Accelerated Filer. Definitions of these categories are stated in Rule 12b-2 of the Exchange Act. This information should be based on the registrant's current or most recent filing containing the related disclosure.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
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| - Definition
State aggregate market value of voting and non-voting common equity held by non-affiliates computed by reference to price at which the common equity was last sold, or average bid and asked price of such common equity, as of the last business day of registrant's most recently completed second fiscal quarter. The public float should be reported on the cover page of the registrants form 10K.
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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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Indicate "Yes" or "No" if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.
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Indicate "Yes" or "No" if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Is used on Form Type: 10-K, 10-Q, 8-K, 20-F, 6-K, 10-K/A, 10-Q/A, 20-F/A, 6-K/A, N-CSR, N-Q, N-1A.
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| X                                                                                                                                                                                                                                                                |
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Trading symbol of an instrument as listed on an exchange.
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