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Document and Entity Information Document - USD ($)
 12 Months Ended
Dec. 31, 2017
Feb. 23, 2018
DOCUMENT AND ENTITY INFORMATION [Abstract]    
Document Type 10-K  
Amendment Flag false  
Document Period End Date Dec. 31, 2017  
Document Fiscal Year Focus 2017  
Document Fiscal Period Focus FY  
Trading Symbol FFBC  
Entity Registrant Name FIRST FINANCIAL BANCORP /OH/  
Entity Central Index Key 0000708955  
Current Fiscal Year End Date --12-31  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   62,093,120
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Public Float $ 1,690,000,000