| **General**                                                                         | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
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| **Organization, Consolidation and Presentation of Financial Statements [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
| General                                                                             | GENERAL Financial Statement PreparationThe significant accounting policies followed by First Merchants Corporation (the “Corporation”) and its wholly owned subsidiaries for interim financial reporting are consistent with the accounting policies followed for annual financial reporting.  All adjustments, which are of a normal recurring nature and are in the opinion of management necessary for a fair statement of the results for the periods reported, have been included in the accompanying consolidated condensed financial statements.The consolidated condensed balance sheet of the Corporation as of December 31, 2014, has been derived from the audited consolidated balance sheet of the Corporation as of that date.  Certain information and note disclosures normally included in the Corporation’s annual financial statements, prepared in accordance with accounting principles generally accepted in the United States of America, have been condensed or omitted.  These consolidated condensed financial statements should be read in conjunction with the consolidated financial statements and notes thereto included in the Corporation’s Annual Report on Form 10-K for the year ended December 31, 2014 filed with the Securities and Exchange Commission.  The results of operations for the three and nine months ended September 30, 2015, are not necessarily indicative of the results to be expected for the year.  Reclassifications have been made to prior financial statements to conform to the current financial statement presentation.  These reclassifications had no effect on net income. |

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No definition available.
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| - Definition
The entire disclosure for organization, consolidation and basis of presentation of financial statements disclosure.
+ References
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