| **Regulatory Capital (Tables)**                                                       | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|---------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Banking and Thrift [Abstract]**                                                     |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
| Schedule of Compliance with Regulatory Capital Requirements under Banking Regulations | 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 |

| X                                                                                                                                                            |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_BankingAndThriftAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the capital amounts and ratios as of the balance sheet date, indicating whether the entity or entities are in compliance with regulatory capital requirements, by entity.
+ References
Reference 1: 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=SL5958577-112826Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-DEP -Paragraph 18 -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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1H -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=7881181&loc=d3e65071-112826Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=7881181&loc=SL5958568-112826Reference 5: 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=SL5958570-112826
+ Details**Name:**us-gaap_ScheduleOfComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |