| **Regulatory Capital**            | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
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| **Banking and Thrift [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Regulatory Capital                | REGULATORY CAPITALThe Corporation and the Bank are subject to various regulatory capital requirements administered by the federal banking agencies and are assigned to a capital category.  The assigned capital category is largely determined by three ratios that are calculated according to the regulations: total risk adjusted capital, Tier 1 capital, and Tier 1 leverage ratios. The ratios are intended to measure capital relative to assets and credit risk associated with those assets and off-balance sheet exposures of the entity.  The capital category assigned to an entity can also be affected by qualitative judgments made by regulatory agencies about the risk inherent in the entity's activities that are not part of the calculated ratios.There are five capital categories defined in the regulations, ranging from well capitalized to critically undercapitalized. Classification of a bank in any of the undercapitalized categories can result in actions by regulators that could have a material effect on a bank's operations.At December 31, 2012, the management of the Corporation believes that it meets all capital adequacy requirements to which it is subject. The most recent notifications from the regulatory agencies categorized the Bank as well capitalized under the regulatory framework for prompt corrective action.  To be categorized as well capitalized, a bank must maintain a minimum total capital to risk-weighted assets, Tier I capital to risk-weighted assets and Tier I capital to average assets of 10 percent, 6 percent and 5 percent, respectively.  It should be noted that a bank's capital category is determined solely for the purpose of applying the OCC's "prompt corrective action" regulations and that the capital category may not constitute an accurate representation of the Bank's overall financial condition or prospects.Actual and required capital amounts and ratios are listed below. 20122011   Required For  Required For ActualAdequate CapitalActualAdequate CapitalDecember 31,AmountRatioAmountRatioAmountRatioAmountRatioTotal Capital (to Risk-weighted Assets)        First Merchants Corporation$526,79216.34%$257,9278.00%$487,39316.54%$235,7818.00%First Merchants Bank515,33716.01257,4468.00477,80516.26235,0898.00Tier I Capital (to Risk-weighted Assets)     First Merchants Corporation$456,13214.15%$128,9644.00%$410,13213.92%$117,8904.00%First Merchants Bank474,78214.75128,7234.00440,90915.00117,5454.00Tier I Capital (to Average Assets)     First Merchants Corporation$456,13211.03%$165,4604.00%$410,13210.17%$161,3504.00%First Merchants Bank474,78211.50165,1244.00440,90910.96160,8484.00 |

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| - Definition
The entire disclosure for banks, savings institutions, and credit unions, for regulatory capital requirements imposed by the Federal Reserve System (FRB), the Federal Deposit Insurance Corporation (FDIC), the Office of Thrift Supervision (OTS) or for any state imposed capital requirements, as applicable.  The disclosure may include (1) a description of regulatory capital requirements (a) for capital adequacy purposes and (b) established by the prompt corrective action provisions of Section 38 of the Federal Depository Insurance Act; (2) the actual or possible material effects of noncompliance with such requirements; (3) whether the entity is in compliance with the regulatory capital requirements including (a) required and actual ratios and amounts of Tier 1 leverage, Tier 1 risk-based, and total risk-based capital, tangible capital (for savings institutions), and Tier 3 capital for market risk (for certain banks and bank holding companies), (b) factors that may significantly affect capital adequacy; (4) the prompt corrective action category in which the entity was classified as of its most recent notification; (5) whether management believes any conditions or events since notification have changed the entity's category. Also may include additional information that might be disclosed in situations where substantial doubt about the entity's ability to continue as a going concern for a reasonable period of time.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1C -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958572-112826Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1D -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958573-112826Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 15, 16, 17 -IssueDate 2006-05-01 -Chapter 17 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1E -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958574-112826Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958570-112826Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1A -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958569-112826Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1F -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958575-112826Reference 10: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1G -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958577-112826
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