BUSINESS AND SIGNIFICANT ACCOUNTING POLICIES: (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Share Based Compensation [Policy Text Block]	Stock based compensation: Compensation cost is recognized for restricted stock awards and units issued to employees based on the fair value of these awards at the date of grant. Market price of the Corporation’s common stock at the date of grant is used for restricted stock awards. Compensation expense is recognized over the requisite service period.
Organization	Organization: The consolidated financial statements of First Financial Corporation and its subsidiaries (the Corporation) include the parent company and its wholly-owned subsidiaries, First Financial Bank, N.A. headquartered in Vigo County, Indiana, The Morris Plan Company of Terre Haute (Morris Plan), First Chanticleer Corporation, a property rental entity headquartered in Terre Haute, Indiana, and FFB Risk Management Co., Inc., a captive insurance subsidiary headquartered in Las Vegas, Nevada. Inter-company transactions and balances have been eliminated.
Use of Estimates	Use of Estimates: To prepare financial statements in conformity with U.S. generally accepted accounting principles, management makes estimates and assumptions based on available information. These estimates and assumptions affect the amounts reported in the financial statements and disclosures provided, and actual results could differ.
Cash Flows	Cash Flows: Cash and cash equivalents include cash and demand deposits with other financial institutions. Net cash flows are reported for customer loan and deposit transactions and short-term borrowings.
Securities	Securities: The Corporation classifies all securities as "available for sale." Securities are classified as available for sale when they might be sold before maturity. Securities available for sale are carried at fair value with unrealized holdings gains and losses, net of taxes, reported in other comprehensive income within shareholders' equity. Interest income includes amortization of purchase premium or discount. Premiums and discounts are amortized on the level yield method without anticipating prepayments. Mortgage-backed securities are amortized over the expected life. Realized gains and losses on sales are based on the amortized cost of the security sold. Management evaluates securities for other-than temporary impairment (OTTI) at least on a quarterly basis, and more frequently when economic or market conditions warrant such an evaluation.
Loans	Loans: Loans that management has the intent and ability to hold for the foreseeable future until maturity or pay-off are reported at the principal balance outstanding, net of unearned interest, purchase premiums and discounts, deferred loan fees and costs, and allowance for loan losses. Loans held for sale are reported at the lower of cost or fair value, on an aggregate basis. Interest income is accrued on the unpaid principal balance and includes amortization of net deferred loan fees and costs over the loan term without anticipating prepayments. The recorded investment in loans includes accrued interest receivable and net deferred loan fees and costs. Interest income is not reported when full loan repayment is in doubt, typically when the loan is impaired or payments are significantly past due. Past-due status is based on the contractual terms of the loan. All interest accrued but not received for loans placed on non-accrual is reversed against interest income. Interest received on such loans is accounted for on the cash-basis or cost-recovery method, until qualifying for return to accrual. Loans are returned to accrual status when all the principal and interest amounts contractually due are brought current and future payments are reasonably assured. In all cases, loans are placed on non-accrual or charged-off if collection of principal or interest is considered doubtful. The above policies are consistent for all segments of loans.
Certain Purchased Loans	Certain Purchased Loans: The Corporation purchases individual loans and groups of loans, some of which have shown evidence of credit deterioration since origination. These purchased loans are recorded at the amount paid, such that there is no carryover of the seller's allowance for loan losses. After acquisition, losses are recognized by an increase in the allowance for loan losses. Such purchased loans are accounted for individually. The Corporation estimates the amount and timing of expected cash flows for each purchased loan, and the expected cash flows in excess of amount paid are recorded as interest income over the remaining life of the loan (accretable yield). The excess of the loan's contractual principal and interest over expected cash flows is not recorded (nonaccretable difference). Over the life of the loan, expected cash flows continue to be estimated. If the present value of expected cash flows is less than the carrying amount, a provision for loan loss is recorded. If the present value of expected cash flows is greater than the carrying amount, it is recognized as part of future interest income.
Concentration of Credit Risk	Concentration of Credit Risk: Most of the Corporation's business activity is with customers located within west central Indiana and east central Illinois. Therefore, the Corporation's exposure to credit risk is significantly affected by changes in the economy of this area. A major economic downturn in this area would have a negative effect on the Corporation's loan portfolio.
Allowance for Loan Losses	Allowance for Loan Losses: The allowance for loan losses is a valuation allowance for probable incurred credit losses. Loan losses are charged against the allowance when management believes the uncollectibility of a loan balance is confirmed. Subsequent recoveries, if any, are credited to the allowance. Management estimates the allowance balance required using past loan loss experience, the nature and volume of the portfolio, information about specific borrower situations and estimated collateral values, economic conditions and other factors. Allocations of the allowance may be made for specific loans, but the entire allowance is available for any loan that, in management's judgment, should be charged off. The allowance consists of specific and general components. The specific component relates to loans that are individually classified as impaired. The general component covers non-classified loans as well as non-impaired classified loans and is based on historical loss experience adjusted for current factors. A loan is impaired when full payment under the loan terms is not expected. Loans for which the terms have been modified, and for which the borrower is experiencing financial difficulties, are considered troubled debt restructurings and classified as impaired. Impairment is evaluated in total for smaller-balance loans of similar nature such as residential mortgages and consumer loans, and on an individual basis for other loans. If a loan is impaired, a portion of the allowance is allocated so that the loan is reported, net, at the present value of estimated future cash flows, using the loan's existing rate, or at the fair value of collateral if repayment is expected solely from the collateral. Large groups of smaller balance homogeneous loans, such as consumer and residential real estate loans, are collectively evaluated for impairment and, accordingly, they are not separately identified for impairment disclosures. The general component covers non-classified loans as well as non-impaired classified loans and is based on historical loss experience adjusted for current factors. The historical loss experience is based on the actual loss history experienced over the most recent four years. This actual loss experience is supplemented with other current factors based on the risks present for each portfolio segment. These current factors include consideration of the following: levels of and trends in delinquent, classified, and impaired loans; levels of and trends in charge-offs and recoveries; national and local economic trends and conditions; changes in lending policies and procedures; trends in volume and terms of loans; experience, ability, and depth of lending management and other relevant staff; credit concentrations; value of underlying collateral for collateral dependent loans; and other external factors such as competition and legal and regulatory requirements. The following portfolio segments have been identified: commercial loans, residential loans and consumer loans. A characteristic of the commercial loan segment is that the loans are for business purchases. A characteristic of the residential loan segment is that the loans are secured by residential properties. A characteristic of the consumer loan segment is that the loans are for automobiles and other consumer purchases. Commercial loans are generally well secured, which mitigates the risk of loss and has contributed to the low historical loss rate. However, concentrations in commercial real estate, along with the potential impact of rising interest rates to commercial real estate, raises the risk of loss on commercial loans. For these reasons, commercial loans have the highest adjustment to the historical loss rate. Continued weakness in local economic conditions along with declining auto values resulted in consumer loans having the next highest level of adjustment to the historical loss rate. The residential loan portfolio segment had the lowest level of adjustment to the historical loss rate. Troubled debt restructurings are separately identified for impairment disclosures and are measured at the present value of estimated future cash flows using the loan's effective rate at inception. If a troubled debt restructuring is considered to be a collateral dependent loan, the loan is reported, net, at the fair value of the collateral. For troubled debt restructurings that subsequently default, the Corporation determines the amount of reserve in accordance with the accounting policy for the allowance for loan losses.
FDIC Indemnification Asset	FDIC Indemnification Asset: The FDIC indemnification asset results from the loss share agreements in the 2009 FDIC-assisted transaction. The asset is measured separately from the related covered assets as they are not contractually embedded in the assets and are not transferable with the assets should the Corporation choose to dispose of them. It represents the acquisition date fair value of expected reimbursements from the FDIC which was determined to be $12.1 million. Pursuant to the terms of the loss sharing agreement, covered loans and other real estate are subject to a stated loss threshold whereby the FDIC will reimburse the Corporation for up to 95% of losses incurred. These expected reimbursements do not include reimbursable amounts related to future covered expenditures. These cash flows are discounted to reflect a metric of uncertainty of the timing and receipt of the loss sharing reimbursement from the FDIC. This asset decreases when losses are realized and claims are paid by the FDIC or when customers repay their loans in full and expected losses do not occur. This asset also increases when estimated future losses increase. When estimated future losses increase, the Corporation records a provision for loan losses and increases its allowance for loan losses accordingly. The related increase or decrease in the FDIC indemnification asset is recorded as an (increase) or offset to the provision for loan losses.
Foreclosed Assets	Foreclosed Assets: Assets acquired through or instead of loan foreclosures are initially recorded at fair value less estimated selling costs when acquired, establishing a new cost basis. Physical possession of residential real estate property collateralizing a consumer mortgage loan occurs when legal title is obtained upon completion of foreclosure or when the borrower conveys all interest in the property to satisfy the loan through completion of a deed in lieu of foreclosure or similar legal agreement. These assets are subsequently accounted for at lower of cost or fair value less estimated costs to sell. If fair value declines, a valuation allowance is recorded through expense. Costs after acquisition are expensed.
Premises and Equipment	Premises and Equipment: Land is carried at cost. Premises and equipment are stated at cost less accumulated depreciation. Depreciation is computed over the useful lives of the assets, which range from 3 to 5 years for furniture and equipment and 33 to 39 years for buildings and leasehold improvements.
Restricted Stock	Restricted Stock: Restricted stock includes Federal Home Loan Bank (FHLB) of Indianapolis and Federal Reserve stock. This restricted stock is carried at cost and periodically evaluated for impairment. Because this stock is viewed as a long-term investment, impairment is based on ultimate recovery of par value. Both cash and stock dividends are reported as income.
Servicing Rights	Servicing Rights: Servicing rights are recognized separately when they are acquired through sales of loans. When mortgage loans are sold, servicing rights are initially recorded at fair value with the income statement effect recorded in gains on sales of loans. Fair value is based on market prices for comparable mortgage servicing contracts, when available, or alternatively, is based on third-party valuations that incorporate assumptions that market participants would use in estimating future net servicing income, such as the cost to service, the discount rate, ancillary income, prepayment speeds and default rates and losses. All classes of servicing assets are subsequently measured using the amortization method, which requires servicing rights to be amortized into non-interest income in proportion to, and over the period of, the estimated future net servicing income of the underlying loans. Servicing assets are evaluated for impairment based upon the fair value of the rights as compared to carrying amount. Impairment is determined by stratifying rights into groupings based on predominant risk characteristics, such as interest rate, loan type and investor type. Impairment is recognized through a valuation allowance for an individual grouping, to the extent that fair value is less than the carrying amount. If the Corporation later determines that all or a portion of the impairment no longer exists for a particular grouping, a reduction of the allowance may be recorded as an increase to income. Changes in valuation allowances are reported with Other Service Charges and Fees on the income statement. The fair values of servicing rights are subject to significant fluctuations as a result of changes in estimated and actual prepayment speeds and default rates and losses. Servicing fee income, which is included in Other Service Charges and Fees on the income statement, is for fees earned for servicing loans. The fees are based on a contractual percentage of the outstanding principal or a fixed amount per loan and are recorded as income when earned. The amortization of mortgage servicing rights is netted against loan servicing fee income.
Transfers of Financial Assets	Transfers of Financial Assets: Transfers of financial assets are accounted for as sales, when control over the assets has been relinquished. Control over transferred assets is deemed to be surrendered when the assets have been isolated from the Corporation, the transferee obtains the right (free of conditions that constrain it from taking advantage of that right) to pledge or exchange the transferred assets, and the Corporation does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity.
Life Insurance Corporate Or Bank Owned Policy [Text Block]	Bank-Owned Life Insurance: The Corporation has purchased life insurance policies on certain key executives. Bank-owned life insurance is recorded at its cash surrender value, or the amount that can be realized. Income on the investments in life insurance is included in other interest income.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets: Goodwill resulting from business combinations prior to January 1, 2009 represents the excess of the purchase price over the fair value of the net assets of businesses acquired. Goodwill resulting from business combinations after January 1, 2009 represents the future economic benefits arising from other assets acquired that are not individually identified and separately recognized. Goodwill and intangible assets acquired in a purchase business combination and determined to have an indefinite useful life are not amortized, but tested for impairment at least annually. The Corporation has selected December 31 as the date to perform the annual impairment test. Intangible assets with definite useful lives are amortized over their estimated useful lives to their estimated residual values. Goodwill is the only intangible asset with an indefinite life on our balance sheet. Other intangible assets consist of core deposit assets arising from the whole bank and branch acquisitions. They are initially measured at fair value and then are amortized on an accelerated basis over their estimated useful lives, which are 10 and 12 years, respectively.
Long-Term Assets	Long-Term Assets: Premises and equipment and other long-term assets are reviewed for impairment when events indicate their carrying amount may not be recoverable from future undiscounted cash flows. If impaired, the assets are recorded at fair value.
Benefit Plans	Benefit Plans: Pension expense is the net of service and interest cost, return on plan assets and amortization of gains and losses not immediately recognized. The amount contributed is determined by a formula as decided by the Board of Directors. Deferred compensation and supplemental retirement plan expense allocates the benefits over years of service.
Employee Stock Ownership Plan	Employee Stock Ownership Plan: Shares of treasury stock are issued to the ESOP and compensation expense is recognized based upon the total market price of shares when contributed.
Deferred Compensation Plan	Deferred Compensation Plan: Prior to 2011, a deferred compensation plan covered all directors. Under the plan, the Corporation pays each director, or their beneficiary, the amount of fees deferred plus interest over 10 years, beginning when the director achieves age 65. A liability is accrued for the obligation under these plans.
Incentive Plans	Incentive Plans: A long-term incentive plan established in 2000 provides for the payment of incentive rewards as a 15-year annuity to all directors and certain key officers. That plan was in place through December 31, 2009, and compensation expense is recognized over the service period. Payments under the plan generally did not begin until the earlier of January 1, 2015, or the January 1 immediately following the year in which the participant reaches age 65. There was no compensation expense related to this plan for 2017, 2016 and 2015. There is a liability of $11.4 million and $12.3 million as of year-end 2017 and 2016. In 2011 the Corporation adopted the 2011 Short-term Incentive Plan and the 2011 Omnibus Equity Incentive Plan designed to reward key officers based on certain performance measures.
Income Taxes	Income Taxes: Income tax expense is the total of the current year income tax due or refundable and the change in deferred tax assets and liabilities. Deferred tax assets and liabilities are the expected future tax amounts for the temporary differences between carrying amounts and tax bases of assets and liabilities, computed using enacted tax rates. A valuation allowance, if needed, reduces deferred tax assets to the amount expected to be realized. A tax position is recognized as a benefit only if it is "more likely than not" that the tax position would be sustained in a tax examination, with a tax examination being presumed to occur. The amount recognized is the largest amount of tax benefit that is greater than 50% likely of being realized on examination. For tax positions not meeting the "more likely than not" test, no tax benefit is recorded. The Corporation recognizes interest and/or penalties related to income tax matters in income tax expense.
Loan Commitments and Related Financial Instruments	Loan Commitments and Related Financial Instruments: Financial instruments include credit instruments, such as commitments to make loans and standby letters of credit, issued to meet customer financing needs. The face amount for these items represents the exposure to loss, before considering customer collateral or ability to repay. Such financial instruments are recorded when they are funded.
Earnings Per Share	Earnings Per Share: Earnings per common share is net income divided by the weighted average number of common shares outstanding during the period. The Corporation does not have any potentially dilutive securities as the restricted stock awards are included in outstanding shares.. Earnings and dividends per share are restated for stock splits and dividends through the date of issue of the financial statements.
Comprehensive Income	Comprehensive Income: Comprehensive income consists of net income and other comprehensive income. Other comprehensive income includes unrealized gains and losses on securities available for sale and changes in the funded status of the retirement plans, net of taxes, which are also recognized as separate components of equity.
Loss Contingencies	Loss Contingencies: Loss contingencies, including claims and legal actions arising in the ordinary course of business, are recorded as liabilities when the likelihood of loss is probable and an amount of range of loss can be reasonably estimated. Management does not believe there are currently such matters that will have a material effect on the financial statements.
Dividend Restriction	Dividend Restriction: Banking regulations require maintaining certain capital levels and may limit the dividends paid by the bank to the holding company or by the holding company to shareholders.
Fair Value of Financial Instruments	Fair Value of Financial Instruments: Fair values of financial instruments are estimated using relevant market information and other assumptions, as more fully disclosed in a separate note. Fair value estimates involve uncertainties and matters of significant judgment regarding interest rates, credit risk, prepayments and other factors, especially in the absence of broad markets for particular items. Changes in assumptions or market conditions could significantly affect the estimates.
Operating Segment	Operating Segment: While the Corporation's chief decision-makers monitor the revenue streams of the various products and services, the operating results of significant segments are similar and operations are managed and financial performance is evaluated on a corporate-wide basis. Accordingly, all of the Corporation's financial service operations are considered by management to be aggregated in one reportable operating segment, which is banking.
Adoption of New Accounting Standards	: In May 2014, the FASB issued an update (ASU No. 2014-09, Revenue from Contracts with Customers) creating FASB Topic 606, Revenue from Contracts with Customers. The guidance in this update affects any entity that either enters into contracts with customers to transfer goods or services or enters into contracts for the transfer of nonfinancial assets unless those contracts are within the scope of other standards (for example, insurance contracts or lease contracts). The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve that core principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract, (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract and (v) recognize revenue when (or as) the entity satisfies a performance obligation. ASU 2015-4 “Revenue from Contracts with Customers - Deferral of the Effective Date” deferred the effective date of ASU 2014-09 by one year and as a result, the new standard will be effective the first quarter of 2018. The Corporation’s revenue is comprised of net interest income on financial assets and financial liabilities, which is explicitly excluded from the scope of ASU 2014-09, and non-interest income. Based on the Corporation’s analysis of the effect of the new standard on its recurring revenue streams, the Corporation did not expect and did not experience an impact on the Corporation’s financial statements upon adoption in the first quarter of 2018. No adjustments to opening retained earnings was recorded on January 1, 2018. In January 2016, the FASB issued ASU 2016-01, Recognition and Measurement of Financial Assets and Financial Liabilities, amending ASU Subtopic 825-10. The amendments in this update make targeted improvements to generally accepted accounting principles (GAAP) as follows: 1) Require equity investments to be measured at fair value with changes in fair value recognized in net income.; 2) Simplify the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment.; 3) Eliminate the requirement to disclose the fair value of financial instruments measured at amortized cost for entities that are not public business entities.; 4) Eliminate the requirement for public business entities to disclose the method(s) and significant assumptions used to estimate the fair value that is required to be disclosed for financial instruments measured at amortized cost on the balance sheet.; 5) Require public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes.; 6) Require an entity to present separately in other comprehensive income the portion of the total change in the fair value of a liability resulting from a change in the instrument-specific credit risk when the entity has elected to measure the liability at fair value in accordance with the fair value option for financial instruments.; 7) Require separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or the accompanying notes to the financial statements.; and 8) Clarify that an entity should evaluate the need for a valuation allowance on a deferred tax asset related to available-for-sale securities in combination with the entity’s other deferred tax assets. The amendments in this update are effective for fiscal years beginning after December 15, 2017. ASU 2016-1 became effective on January 1, 2018 and did not have a significant impact on the Corporation’s financial statements. However, the fair value disclosures for our loan portfolio will consider the exit price. In February 2016, the FASB issued ASU No. 2016-02, "Leases." Under the new guidance, lessees will be required to recognize the following for all leases (with the exception of short-term leases): 1) a lease liability, which is the present value of a lessee's obligation to make lease payments, and 2) a right-of-use asset, which is an asset that represents the lessee's right to use, or control the use of, a specified asset for the lease term. Lessor accounting under the new guidance remains largely unchanged as it is substantially equivalent to existing guidance for sales-type leases, direct financing leases, and operating leases. Leveraged leases have been eliminated, although lessors can continue to account for existing leveraged leases using the current accounting guidance. Other limited changes were made to align lessor accounting with the lessee accounting model and the new revenue recognition standard. All entities will classify leases to determine how to recognize lease-related revenue and expense. Quantitative and qualitative disclosures will be required by lessees and lessors to meet the objective of enabling users of financial statements to assess the amount, timing, and uncertainty of cash flows arising from leases. The intention is to require enough information to supplement the amounts recorded in the financial statements so that users can understand more about the nature of an entity’s leasing activities. ASU No. 2016-02 is effective for interim and annual reporting periods beginning after December 15, 2018; early adoption is permitted. All entities are required to use a modified retrospective approach for leases that exist or are entered into after the beginning of the earliest comparative period in the financial statements. They have the option to use certain relief; full retrospective application is prohibited. The Corporation continues to evaluate the provision of the new lease standard but, due to the small number of lease agreements presently in effect for the Corporation, does not expect the new guidance will have a significant impact on the Corporation’s financial statements. In June 2016 ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments (ASU 2016-13), was issued and requires entities to use a current expected credit loss ("CECL") model which is a new impairment model based on expected losses rather than incurred losses. Under this model an entity would recognize an impairment allowance equal to its current estimate of all contractual cash flows that the entity does not expect to collect from financial assets measured at amortized cost. The entity's estimate would consider relevant information about past events, current conditions, and reasonable and supportable forecasts, which will result in recognition of lifetime expected credit losses upon loan origination. ASU 2016-13 is effective for interim and annual reporting periods beginning after December 15, 2019, with early adoption permitted for annual reporting periods beginning after December 15, 2018. Management has initiated an implementation committee to assist in assessing data and system needs for the new standard. Management anticipates the effect will be an increase to the allowance for loan losses upon adoption, however, the overall increase is uncertain at this time. In August of 2016 ASU 2016-15 "Statement of Cash Flows (Topic 230)" ("ASU 2016-15") was issued and is intended to reduce the diversity in practice around how certain transactions are classified within the statement of cash flows. ASU 2016-15 is effective for public companies for annual periods beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted with retrospective application. ASU 2016-15 became effective on January 1, 2018 and did not have a significant impact on the Corporation’s accounting and disclosures. In January 2017, the FASB issued ASU No. 2017-04, “Simplifying the Test for Goodwill Impairment.” The guidance removes Step 2 of the goodwill impairment test, which requires a hypothetical purchase price allocation. Goodwill impairment will now be the amount by which a reporting unit’s carrying value exceeds its fair value, not to exceed the carrying amount of goodwill. All other goodwill impairment guidance will remain largely unchanged. ASU No. 2017-04 is effective for interim and annual reporting periods beginning after December 15, 2019, applied prospectively. Early adoption is permitted for any impairment tests performed after January 1, 2017. The Corporation is assessing ASU 2017-04 but does not expect a significant impact on its accounting and disclosures. In March 2017, the FASB issued ASU No. 2017-07, “Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost.” Under the new guidance, employers will present the service cost component of the net periodic benefit cost in the same income statement line item (e.g., Salaries and Benefits) as other employee compensation costs arising from services rendered during the period. In addition, only the service cost component will be eligible for capitalization in assets. Employers will present the other components separately (e.g., Other Noninterest Expense) from the line item that includes the service cost. ASU No. 2017-07 is effective for interim and annual reporting periods beginning after December 15, 2017. Early adoption is permitted, however, the Corporation has decided not to early adopt. Employers will apply the guidance on the presentation of the components of net periodic benefit cost in the income statement retrospectively. The guidance limiting the capitalization of net periodic benefit cost in assets to the service cost component will be applied prospectively. ASU 2017-07 became effective on January 1, 2018 and did not have a significant impact on the Corporation’s accounting and disclosures. In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification”. ASU 2017-09 was issued to provide clarity and reduce both 1) diversity in practice and 2) cost and complexity when applying the guidance in Topic 718, Compensation - Stock Compensation, to a change to the terms or conditions of a share-based payment award. Diversity in practice has arisen in part because some entities apply modification accounting under Topic 718 for modifications to terms and conditions that they consider substantive, but do not when they conclude that particular modifications are not substantive. Others apply modification accounting for any change to an award, except for changes that they consider purely administrative in nature. Still others apply modification accounting when a change to an award changes the fair value, the vesting, or the classification of the award. In practice, it appears that the evaluation of a change in fair value, vesting, or classification may be used to evaluate whether a change is substantive. ASU 2017-09 include guidance on determining which changes to the terms and conditions of share-based payment awards require an entity to apply modification accounting under Topic 718. ASU 2017-09 is effective for the annual period, and interim periods within the annual periods, beginning after December 15, 2017. Early adoption is permitted, including adoption in any interim period for: (a) public business entities for reporting periods for which financial statements have not yet been issued, and (b) all other entities for reporting periods for which financial statements have not yet been made available for issuance. ASU 2017-09 should be applied prospectively to an award modified on or after the adoption date.