| Document and Entity Information Document | Aug. 29, 2019 |
| Cover page. | |
| Document Type | 8-K |
| Document Period End Date | Aug. 29, 2019 |
| Entity Registrant Name | FIRST FINANCIAL BANCORP. |
| Entity Incorporation, State or Country Code | OH |
| Entity File Number | 001-34762 |
| Entity Tax Identification Number | 31-1042001 |
| Entity Address, Address Line One | 255 East Fifth Street, Suite 800 |
| Entity Address, City or Town | Cincinnati, |
| Entity Address, State or Province | OH |
| Entity Address, Postal Zip Code | 45202 |
| City Area Code | 877 |
| Local Phone Number | 322-9530 |
| 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 | 0000708955 |
| Amendment Flag | false |
| NASDAQ/NMS (GLOBAL MARKET) [Member] | |
| Entity Listings [Line Items] | |
| Title of 12(b) Security | Common stock, No par value |
| Trading Symbol | FFBC |
| Security Exchange Name | NASDAQ |
