SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
Sep. 30, 2018
Off-Balance-Sheet Credit Exposure, Policy [Policy Text Block]	Letters of credit are conditional commitments issued by First Financial to guarantee the performance of a client to a third party. First Financial’s portfolio of letters of credit consists primarily of performance assurances made on behalf of clients who have a contractual commitment to produce or deliver goods or services. The risk to First Financial arises from its obligation to make payment in the event of the client's contractual default to produce the contracted good or service to a third party.
Basis of Presentation Policy	The Consolidated Financial Statements of First Financial Bancorp., a financial holding company principally serving Ohio, Indiana, Kentucky and Illinois, include the accounts and operations of First Financial and its wholly-owned subsidiary, First Financial Bank. All significant intercompany transactions and accounts have been eliminated in consolidation. Certain reclassifications of prior periods' amounts have been made to conform to current year presentation. Such reclassifications had no effect on net earnings.
Use of Estimates, Policy	The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the Consolidated Financial Statements and accompanying Notes. These estimates, assumptions and judgments are inherently subjective and may be susceptible to significant change. Actual realized amounts could differ materially from these estimates.
Loans and Leases Receivable, Past Due Status, Policy	Loans are considered past due or delinquent when the contractual principal or interest due in accordance with the terms of the loan agreement remains unpaid after the date of the scheduled payment.
Loans and Leases Receivable, Nonaccrual Loan and Lease Status, Policy	Loans are classified as nonaccrual when, in the opinion of management, collection of principal or interest is doubtful or when principal or interest payments are 90 days or more past due. Generally, loans are classified as nonaccrual due to the continued failure by the borrower to adhere to contractual payment terms, coupled with other pertinent factors. When a loan is classified as nonaccrual, the accrual of interest income is discontinued and previously accrued but unpaid interest is reversed. Any payments received while a loan is on nonaccrual status are applied as a reduction to the carrying value of the loan. A loan classified as nonaccrual may return to accrual status if collection of future principal and interest payments is no longer doubtful. Purchased impaired loans are classified as performing, even though they may be contractually past due, as any nonpayment of contractual principal or interest is considered in the periodic re-estimation of expected cash flows and is included in the resulting recognition of current period provision for loan and lease losses or prospective yield adjustments.
Loans and Leases Receivable, Troubled Debt Restructuring Policy	A loan modification is considered a TDR when the borrower is experiencing financial difficulty and concessions are made by the Company that would not otherwise be considered for a borrower with similar credit characteristics. The most common types of modifications include interest rate reductions, maturity extensions and modifications to principal amortization, including interest-only structures. Modified terms are dependent upon the financial position and needs of the individual borrower. If the modification agreement is violated, the loan is managed by the Company’s credit administration group for resolution, which may result in foreclosure in the case of real estate. TDRs are generally classified as nonaccrual for a minimum period of six months and may qualify for return to accrual status once they have demonstrated performance with the restructured terms of the loan agreement.
Impaired Financing Receivable, Policy	First Financial individually reviews all impaired commercial loan relationships, as well as consumer loan TDRs, greater than $250,000, to determine if a specific allowance is necessary based on the borrower’s overall financial condition, payment record, support from guarantors and the realizable value of any collateral. Specific allowances are based on discounted cash flows using the loan's initial effective interest rate or the fair value of the collateral for certain collateral dependent loans.Loans classified as nonaccrual and loans modified as TDRs are considered impaired.
Loans and Leases Receivable, Real Estate Acquired Through Foreclosure, Policy	OREO consists of properties acquired by the Company primarily through the loan foreclosure or repossession process, that results in partial or total satisfaction of problem loans.
Loans and Leases Receivable, Allowance for Loan Losses Policy	Management maintains the ALLL at a level that it considers sufficient to absorb probable incurred loan and lease losses inherent in the portfolio. Management determines the adequacy of the ALLL based on historical loss experience as well as other significant factors such as composition of the portfolio, economic conditions, geographic footprint, the results of periodic internal and external evaluations of delinquent, nonaccrual and classified loans and any other adverse situations that may affect a specific borrower's ability to repay, including the timing of future payments. The ALLL is increased by provision expense and decreased by charge-offs, net of recoveries of amounts previously charged-off. First Financial's policy is to charge-off all or a portion of a loan when, in management's opinion, it is unlikely to collect the principal amount owed in full either through payments from the borrower or from the liquidation of collateral.
Goodwill and Intangible Assets, Goodwill, Policy	Assets and liabilities acquired in a business combination are recorded at their estimated fair values as of the acquisition date. The excess cost of the acquisition over the fair value of net assets acquired is recorded as goodwill.
Goodwill and Intangible Assets, Goodwill Impairment Policy	Goodwill is evaluated for impairment on an annual basis as of October 1 of each year, or whenever events or changes in circumstances indicate that the fair value of a reporting unit may be below its carrying value.
Goodwill and Intangible Assets, Policy	Core deposit intangibles represent the estimated fair value of acquired customer deposit relationships on the date of acquisition and are amortized on an accelerated basis over their estimated useful lives.
Income Tax, Policy	The unrecognized tax benefits relate to state income tax exposures from taking tax positions where the Company believes it is likely that, upon examination, a state may take a position contrary to the position taken by First Financial.
Commitments and Contingencies, Policy	First Financial offers a variety of financial instruments including standby letters of credit and outstanding commitments to extend credit to assist clients in meeting their requirement for liquidity and credit enhancement. GAAP does not require these financial instruments to be recorded in the Consolidated Financial Statements. First Financial utilizes the same credit policies in issuing commitments and conditional obligations as it does for credit instruments recorded on the Consolidated Balance Sheets. First Financial’s exposure to credit loss in the event of nonperformance by the counterparty is represented by the contractual amounts of those instruments. First Financial utilizes the ALLL methodology to maintain a reserve that it considers sufficient to absorb probable incurred losses inherent in letters of credit and outstanding loan commitments.
Fair Value Measurement, Policy [Policy Text Block]	The fair value framework as disclosed in the Fair Value Topic includes a hierarchy which focuses on prioritizing the inputs used in valuation techniques. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (Level 1), a lower priority to observable inputs other than quoted prices in active markets for identical assets and liabilities (Level 2) and the lowest priority to unobservable inputs (Level 3). When determining the fair value measurements for assets and liabilities, First Financial looks to active markets to price identical assets or liabilities whenever possible and classifies such items in Level 1. When identical assets and liabilities are not traded in active markets, First Financial looks to observable market data for similar assets and liabilities and classifies such items as Level 2. Certain assets and liabilities are not actively traded in observable markets and First Financial must use alternative techniques, based on unobservable inputs, to determine the fair value and classifies such items as Level 3. The level within the fair value hierarchy is based on the lowest level of input that is significant in the fair value measurement.
Fair Value of Financial Instruments, Policy	The following methods, assumptions and valuation techniques were used by First Financial to measure different financial assets and liabilities at fair value on a recurring or nonrecurring basis. Investment securities. Investment securities classified as available-for-sale are recorded at fair value on a recurring basis. Fair value measurement is based upon quoted market prices, when available (Level 1). If quoted market prices are not available, fair values are measured utilizing independent valuation techniques of identical or similar investment securities. First Financial compiles prices from various sources who may apply such techniques as matrix pricing to determine the value of identical or similar investment securities (Level 2). Matrix pricing is a mathematical technique widely used in the banking industry to value investment securities without relying exclusively on quoted prices for the specific investment securities but rather relying on the investment securities’ relationship to other benchmark quoted investment securities. Any investment securities not valued based upon the methods previously described are considered Level 3. First Financial utilizes values provided by third-party pricing vendors to price the investment securities portfolio in accordance with the fair value hierarchy of the Fair Value Topic and reviews the pricing methodologies utilized by the pricing vendors to ensure that the fair value determination is consistent with the applicable accounting guidance. First Financial’s pricing process includes a series of quality assurance activities where prices are compared to recent market conditions, historical prices and other independent pricing services. Further, the Company periodically validates the fair value of a sample of securities in the portfolio by comparing the fair values to prices from other independent sources for the same or similar securities. First Financial analyzes unusual or significant variances, conducts additional research with the pricing vendor, and if necessary, takes appropriate action based on its findings. The results of the quality assurance process are incorporated into the selection of pricing providers by the portfolio manager. Impaired loans. The fair value of impaired loans are specifically reviewed for purposes of determining the appropriate amount of impairment to be allocated to the ALLL. Fair value is generally measured based on the value of the collateral securing the loans. Collateral may be in the form of real estate or business assets including equipment, inventory and accounts receivable. The value of real estate collateral is determined utilizing an income or market valuation approach based on an appraisal conducted by an independent, licensed third-party appraiser (Level 3). The value of business equipment is based on an outside appraisal, if deemed significant, or the net book value on the applicable borrower financial statements. Likewise, values for inventory and accounts receivable collateral are based on borrower financial statement balances or aging reports on a discounted basis as appropriate (Level 3). Impaired loans are measured at fair value on a nonrecurring basis. Any fair value adjustments are recorded in the period incurred as provision for loan and lease losses on the Consolidated Statements of Income. OREO. Assets acquired through loan foreclosure are recorded at fair value less costs to sell, with any difference between the fair value of the property and the carrying value of the loan recorded as a charge-off. If the fair value is higher than the carrying amount of the loan, the excess is recognized first as a recovery and then as noninterest income. Subsequent changes in value are reported as adjustments to the carrying amount and are recorded in noninterest expense. The carrying value of OREO is not re-measured to fair value on a recurring basis, but is subject to fair value adjustments when the carrying value differs from the fair value, less estimated selling costs. Fair value is based on recent real estate appraisals and is updated at least annually. The Company classifies OREO in level 3 of the fair value hierarchy. Derivatives. The fair values of derivative instruments are based primarily on a net present value calculation of the cash flows related to the interest rate swaps at the reporting date, using primarily observable market inputs such as interest rate yield curves which represents the cost to terminate the swap if First Financial should choose to do so. Additionally, First Financial utilizes an internally-developed model to value the credit risk component of derivative assets and liabilities, which is recorded as an adjustment to the fair value of the derivative asset or liability on the reporting date. Derivative instruments are classified as Level 2 in the fair value hierarchy.
Loan Commitments, Policy [Policy Text Block]	Loan commitments are agreements to extend credit to a client, absent any violation of conditions established in the commitment agreement. Commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. The amount of collateral obtained, if deemed necessary by First Financial upon extension of credit, is based on management’s credit evaluation of the client. The collateral held varies, but may include securities, real estate, inventory, plant or equipment.
Other Contract-Mortgage
Derivatives, Methods of Accounting, Hedging Derivatives, Policy	First Financial enters into IRLCs and forward commitments for the future delivery of mortgage loans to third party investors, which are considered derivatives. When borrowers secure an IRLC with First Financial and the loan is intended to be sold, First Financial will enter into forward commitments for the future delivery of the loans to third party investors in order to hedge against the effect of changes in interest rates impacting IRLCs and loans held for sale.
Credit Risk
Derivatives, Methods of Accounting, Hedging Derivatives, Policy	First Financial manages market value credit risk through counterparty credit policies, which require the Company to maintain a total derivative notional position of less than 35% of assets, total credit exposure of less than 3% of capital and no single counterparty credit risk exposure greater than $20.0 million.
Fair Value Hedges
Derivatives, Methods of Accounting, Hedging Derivatives, Policy	First Financial utilizes interest rate swaps as a means to offer commercial borrowers fixed rate funding while providing the Company with floating rate assets.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)

9 Months Ended

Sep. 30, 2018

Off-Balance-Sheet Credit Exposure, Policy [Policy Text Block]

Letters of credit are conditional commitments issued by First Financial to guarantee the performance of a client to a third party. First Financial’s portfolio of letters of credit consists primarily of performance assurances made on behalf of clients who have a contractual commitment to produce or deliver goods or services. The risk to First Financial arises from its obligation to make payment in the event of the client's contractual default to produce the contracted good or service to a third party.

Basis of Presentation Policy

The Consolidated Financial Statements of First Financial Bancorp., a financial holding company principally serving Ohio, Indiana, Kentucky and Illinois, include the accounts and operations of First Financial and its wholly-owned subsidiary, First Financial Bank. All significant intercompany transactions and accounts have been eliminated in consolidation. Certain reclassifications of prior periods' amounts have been made to conform to current year presentation. Such reclassifications had no effect on net earnings.

Use of Estimates, Policy

The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the Consolidated Financial Statements and accompanying Notes. These estimates, assumptions and judgments are inherently subjective and may be susceptible to significant change. Actual realized amounts could differ materially from these estimates.

Loans and Leases Receivable, Past Due Status, Policy

Loans are considered past due or delinquent when the contractual principal or interest due in accordance with the terms of the loan agreement remains unpaid after the date of the scheduled payment.

Loans and Leases Receivable, Nonaccrual Loan and Lease Status, Policy

Loans are classified as nonaccrual when, in the opinion of management, collection of principal or interest is doubtful or when principal or interest payments are 90 days or more past due. Generally, loans are classified as nonaccrual due to the continued failure by the borrower to adhere to contractual payment terms, coupled with other pertinent factors. When a loan is classified as nonaccrual, the accrual of interest income is discontinued and previously accrued but unpaid interest is reversed. Any payments received while a loan is on nonaccrual status are applied as a reduction to the carrying value of the loan. A loan classified as nonaccrual may return to accrual status if collection of future principal and interest payments is no longer doubtful.

Purchased impaired loans are classified as performing, even though they may be contractually past due, as any nonpayment of contractual principal or interest is considered in the periodic re-estimation of expected cash flows and is included in the resulting recognition of current period provision for loan and lease losses or prospective yield adjustments.

Loans and Leases Receivable, Troubled Debt Restructuring Policy

A loan modification is considered a TDR when the borrower is experiencing financial difficulty and concessions are made by the Company that would not otherwise be considered for a borrower with similar credit characteristics. The most common types of modifications include interest rate reductions, maturity extensions and modifications to principal amortization, including interest-only structures. Modified terms are dependent upon the financial position and needs of the individual borrower. If the modification agreement is violated, the loan is managed by the Company’s credit administration group for resolution, which may result in foreclosure in the case of real estate.

TDRs are generally classified as nonaccrual for a minimum period of six months and may qualify for return to accrual status once they have demonstrated performance with the restructured terms of the loan agreement.

Impaired Financing Receivable, Policy

First Financial individually reviews all impaired commercial loan relationships, as well as consumer loan TDRs, greater than $250,000, to determine if a specific allowance is necessary based on the borrower’s overall financial condition, payment record, support from guarantors and the realizable value of any collateral. Specific allowances are based on discounted cash flows using the loan's initial effective interest rate or the fair value of the collateral for certain collateral dependent loans.Loans classified as nonaccrual and loans modified as TDRs are considered impaired.

Loans and Leases Receivable, Real Estate Acquired Through Foreclosure, Policy

OREO consists of properties acquired by the Company primarily through the loan foreclosure or repossession process, that results in partial or total satisfaction of problem loans.

Loans and Leases Receivable, Allowance for Loan Losses Policy

Management maintains the ALLL at a level that it considers sufficient to absorb probable incurred loan and lease losses inherent in the portfolio. Management determines the adequacy of the ALLL based on historical loss experience as well as other significant factors such as composition of the portfolio, economic conditions, geographic footprint, the results of periodic internal and external evaluations of delinquent, nonaccrual and classified loans and any other adverse situations that may affect a specific borrower's ability to repay, including the timing of future payments.

The ALLL is increased by provision expense and decreased by charge-offs, net of recoveries of amounts previously charged-off. First Financial's policy is to charge-off all or a portion of a loan when, in management's opinion, it is unlikely to collect the principal amount owed in full either through payments from the borrower or from the liquidation of collateral.

Goodwill and Intangible Assets, Goodwill, Policy

Assets and liabilities acquired in a business combination are recorded at their estimated fair values as of the acquisition date. The excess cost of the acquisition over the fair value of net assets acquired is recorded as goodwill.

Goodwill and Intangible Assets, Goodwill Impairment Policy

Goodwill is evaluated for impairment on an annual basis as of October 1 of each year, or whenever events or changes in circumstances indicate that the fair value of a reporting unit may be below its carrying value.

Goodwill and Intangible Assets, Policy

Core deposit intangibles represent the estimated fair value of acquired customer deposit relationships on the date of acquisition and are amortized on an accelerated basis over their estimated useful lives.

Income Tax, Policy

The unrecognized tax benefits relate to state income tax exposures from taking tax positions where the Company believes it is likely that, upon examination, a state may take a position contrary to the position taken by First Financial.

Commitments and Contingencies, Policy

First Financial offers a variety of financial instruments including standby letters of credit and outstanding commitments to extend credit to assist clients in meeting their requirement for liquidity and credit enhancement. GAAP does not require these financial instruments to be recorded in the Consolidated Financial Statements.

First Financial utilizes the same credit policies in issuing commitments and conditional obligations as it does for credit instruments recorded on the Consolidated Balance Sheets. First Financial’s exposure to credit loss in the event of nonperformance by the counterparty is represented by the contractual amounts of those instruments. First Financial utilizes the ALLL methodology to maintain a reserve that it considers sufficient to absorb probable incurred losses inherent in letters of credit and outstanding loan commitments.

Fair Value Measurement, Policy [Policy Text Block]

The fair value framework as disclosed in the Fair Value Topic includes a hierarchy which focuses on prioritizing the inputs used in valuation techniques. The fair value hierarchy gives the highest priority to quoted prices in active markets for identical assets or liabilities (Level 1), a lower priority to observable inputs other than quoted prices in active markets for identical assets and liabilities (Level 2) and the lowest priority to unobservable inputs (Level 3). When determining the fair value measurements for assets and liabilities, First Financial looks to active markets to price identical assets or liabilities whenever possible and classifies such items in Level 1. When identical assets and liabilities are not traded in active markets, First Financial looks to observable market data for similar assets and liabilities and classifies such items as Level 2. Certain assets and liabilities are not actively traded in observable markets and First Financial must use alternative techniques, based on unobservable inputs, to determine the fair value and classifies such items as Level 3. The level within the fair value hierarchy is based on the lowest level of input that is significant in the fair value measurement.

Fair Value of Financial Instruments, Policy

The following methods, assumptions and valuation techniques were used by First Financial to measure different financial assets and liabilities at fair value on a recurring or nonrecurring basis.

Investment securities. Investment securities classified as available-for-sale are recorded at fair value on a recurring basis. Fair value measurement is based upon quoted market prices, when available (Level 1). If quoted market prices are not available, fair values are measured utilizing independent valuation techniques of identical or similar investment securities. First Financial compiles prices from various sources who may apply such techniques as matrix pricing to determine the value of identical or similar investment securities (Level 2). Matrix pricing is a mathematical technique widely used in the banking industry to value investment securities without relying exclusively on quoted prices for the specific investment securities but rather relying on the investment securities’ relationship to other benchmark quoted investment securities. Any investment securities not valued based upon the methods previously described are considered Level 3.

First Financial utilizes values provided by third-party pricing vendors to price the investment securities portfolio in accordance with the fair value hierarchy of the Fair Value Topic and reviews the pricing methodologies utilized by the pricing vendors to ensure that the fair value determination is consistent with the applicable accounting guidance. First Financial’s pricing process includes a series of quality assurance activities where prices are compared to recent market conditions, historical prices and other independent pricing services. Further, the Company periodically validates the fair value of a sample of securities in the portfolio by comparing the fair values to prices from other independent sources for the same or similar securities. First Financial analyzes unusual or significant variances, conducts additional research with the pricing vendor, and if necessary, takes appropriate action based on its findings. The results of the quality assurance process are incorporated into the selection of pricing providers by the portfolio manager.

Impaired loans. The fair value of impaired loans are specifically reviewed for purposes of determining the appropriate amount of impairment to be allocated to the ALLL. Fair value is generally measured based on the value of the collateral securing the loans. Collateral may be in the form of real estate or business assets including equipment, inventory and accounts receivable. The value of real estate collateral is determined utilizing an income or market valuation approach based on an appraisal conducted by an independent, licensed third-party appraiser (Level 3). The value of business equipment is based on an outside appraisal, if deemed significant, or the net book value on the applicable borrower financial statements. Likewise, values for inventory and accounts receivable collateral are based on borrower financial statement balances or aging reports on a discounted basis as appropriate (Level 3). Impaired loans are measured at fair value on a nonrecurring basis. Any fair value adjustments are recorded in the period incurred as provision for loan and lease losses on the Consolidated Statements of Income.

OREO. Assets acquired through loan foreclosure are recorded at fair value less costs to sell, with any difference between the fair value of the property and the carrying value of the loan recorded as a charge-off. If the fair value is higher than the carrying amount of the loan, the excess is recognized first as a recovery and then as noninterest income. Subsequent changes in value are reported as adjustments to the carrying amount and are recorded in noninterest expense. The carrying value of OREO is not re-measured to fair value on a recurring basis, but is subject to fair value adjustments when the carrying value differs from the fair value, less estimated selling costs. Fair value is based on recent real estate appraisals and is updated at least annually. The Company classifies OREO in level 3 of the fair value hierarchy.

Derivatives. The fair values of derivative instruments are based primarily on a net present value calculation of the cash flows related to the interest rate swaps at the reporting date, using primarily observable market inputs such as interest rate yield curves which represents the cost to terminate the swap if First Financial should choose to do so. Additionally, First Financial utilizes an internally-developed model to value the credit risk component of derivative assets and liabilities, which is recorded as an adjustment to the fair value of the derivative asset or liability on the reporting date. Derivative instruments are classified as Level 2 in the fair value hierarchy.

Loan Commitments, Policy [Policy Text Block]

Loan commitments are agreements to extend credit to a client, absent any violation of conditions established in the commitment agreement. Commitments generally have fixed expiration dates or other termination clauses and may require payment of a fee. Since many of the commitments are expected to expire without being drawn upon, the total commitment amounts do not necessarily represent future cash requirements. The amount of collateral obtained, if deemed necessary by First Financial upon extension of credit, is based on management’s credit evaluation of the client. The collateral held varies, but may include securities, real estate, inventory, plant or equipment.

Other Contract-Mortgage

Derivatives, Methods of Accounting, Hedging Derivatives, Policy

First Financial enters into IRLCs and forward commitments for the future delivery of mortgage loans to third party investors, which are considered derivatives. When borrowers secure an IRLC with First Financial and the loan is intended to be sold, First Financial will enter into forward commitments for the future delivery of the loans to third party investors in order to hedge against the effect of changes in interest rates impacting IRLCs and loans held for sale.

Credit Risk

Derivatives, Methods of Accounting, Hedging Derivatives, Policy

First Financial manages market value credit risk through counterparty credit policies, which require the Company to maintain a total derivative notional position of less than 35% of assets, total credit exposure of less than 3% of capital and no single counterparty credit risk exposure greater than $20.0 million.

Fair Value Hedges

Derivatives, Methods of Accounting, Hedging Derivatives, Policy

First Financial utilizes interest rate swaps as a means to offer commercial borrowers fixed rate funding while providing the Company with floating rate assets.