| **Acquisitions and Divestitures**    | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  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| **Business Combinations [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 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| Acquisitions and Divestitures        | ACQUISITIONS AND DIVESTITURESAcquisition - Ameriana Bancorp, Inc.On December 31, 2015, the Corporation acquired 100 percent of Ameriana Bancorp, Inc. ("Ameriana").  Ameriana merged with and into the Corporation (the "Ameriana Merger") whereupon the separate corporate existence of Ameriana ceased and the Corporation survived.  Immediately following the Ameriana Merger, Ameriana Bank, an Indiana bank and wholly-owned subsidiary of Ameriana, merged with and into First Merchants Bank, National Association, a national bank and wholly-owned subsidiary of the Corporation (the "Bank"), with the Bank continuing as the surviving bank.  Ameriana was headquartered in New Castle, Indiana and had 13 banking centers serving central and east central Indiana.  Pursuant to the merger agreement, each Ameriana shareholder received 0.9037 shares of the Corporation's common stock for each outstanding share of Ameriana common stock held.  The Corporation issued approximately 2.8 million shares of common stock, which was valued at approximately $70.4 million.  The Corporation engaged in this transaction with the expectation that it would be accretive and expand the existing footprint in central and east central Indiana.  Goodwill resulted from this transaction due to the expected synergies and economies of scale that are expected.Under the acquisition method of accounting, the total purchase price is allocated to net tangible and intangible assets based on their current estimated fair values on the date of the acquisition.   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 $70,402Of the total purchase price, $5,342,000 has been allocated to a core deposit intangible that will be amortized over its estimated life of 10 years.  The remaining purchase price has been allocated to goodwill, which is not deductible for tax purposes.Acquisition - C Financial CorporationOn April 17, 2015, the Corporation acquired 100 percent of C Financial Corporation, ("C Financial").  C Financial merged with and into the Corporation (the “C Financial Merger”) whereupon the separate corporate existence of C Financial ceased and the Corporation survived.  Immediately following the C Financial Merger, Cooper State Bank, an Ohio state bank and wholly-owned subsidiary of C Financial, merged with and into the Bank, with the Bank continuing as the surviving bank.  C Financial was headquartered in Columbus, Ohio and had 6 full service banking centers serving the Columbus, Ohio market.  As part of the $14.5 million C Financial Merger, shareholders of C Financial received $6.738 in cash for each share of C Financial common stock held.  The Corporation expects the transaction to be accretive to income and expand the existing footprint in Columbus, Ohio. Goodwill resulted from this transaction due to the synergies and economies of scale that were expected.  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 $14,500Of the total purchase price, $981,000 has been allocated to a core deposit intangible that will be amortized over its estimated life of 10 years.  The remaining purchase price has been allocated to goodwill, which is deductible over a 15 year period for tax purposes as the transaction was considered a taxable exchange.Subsidiary Divestiture - First Merchants Insurance Services, Inc.On June 12, 2015, the Corporation sold all of its stock in First Merchants Insurance Services, Inc., an Indiana corporation ("FMIG"), to USI Insurance Services LLC, a Delaware limited liability company.  The sale price was $18 million, of which $16 million was paid at closing with the remaining $2 million paid through a two-year promissory note.  The sale of FMIG generated a gain on sale of $8.3 million. |

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| - Definition
The entire disclosure for business combinations, including leverage buyout transactions (as applicable), and divestitures.  This may include a description of a business combination or divestiture (or series of individually immaterial business combinations or divestitures) completed during the period, including background, timing, and assets and liabilities recognized and reclassified or sold. This element does not include fixed asset sales and plant closings.
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