| Document and Entity Information Document | Oct. 28, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 28, 2020 |
| Entity File Number | 0-17071 |
| Entity Registrant Name | FIRST MERCHANTS CORP |
| Entity Incorporation, State or Country Code | IN |
| Entity Tax Identification Number | 35-1544218 |
| Entity Address, Address Line One | 200 East Jackson Street |
| Entity Address, Address Line Two | P.O. Box 792 |
| Entity Address, City or Town | Muncie |
| Entity Address, State or Province | IN |
| Entity Address, Postal Zip Code | 47305-2814 |
| City Area Code | 765 |
| Local Phone Number | 747-1500 |
| Title of 12(b) Security | Common Stock, $0.125 stated value per share |
| Trading Symbol | FRME |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000712534 |
| Amendment Flag | false |
