SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	6 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Jun. 30, 2017
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation Our accompanying consolidated financial statements and these notes to the financial statements include subsidiaries in which we have a controlling financial interest. We own and operate FNBPA, First National Trust Company, First National Investment Services Company, LLC, F.N.B. Investment Advisors, Inc., First National Insurance Agency, LLC (FNIA), Regency, Bank Capital Services, LLC and F.N.B. Capital Corporation, LLC, and include results for each of these entities in the accompanying consolidated financial statements. The accompanying consolidated financial statements include all adjustments that are necessary, in the opinion of management, to fairly reflect our financial position and results of operations in accordance with U.S. generally accepted accounting principles (GAAP). All significant intercompany balances and transactions have been eliminated. Certain prior period amounts have been reclassified to conform to the current period presentation. Such reclassifications had no impact on our net income and stockholders’ equity. Events occurring subsequent to the date of the June 30, 2017 balance sheet have been evaluated for potential recognition or disclosure in the consolidated financial statements through the date of the filing of the consolidated financial statements with the Securities and Exchange Commission (SEC). Certain information and note disclosures normally included in consolidated financial statements prepared in accordance with GAAP have been condensed or omitted pursuant to the rules and regulations of the SEC. The interim operating results are not necessarily indicative of operating results FNB expects for the full year. These interim unaudited consolidated financial statements should be read in conjunction with the audited consolidated financial statements and notes thereto included in FNB’s Annual Report on Form 10-K filed with the SEC on February 23, 2017. The accounting policies presented below have been added or amended for newly material items or the adoption of new accounting standards.
Use of Estimates	Use of Estimates Our accounting and reporting policies conform with GAAP. The preparation of financial statements in conformity with GAAP requires us to make estimates and assumptions that affect the amounts reported in the consolidated financial statements and accompanying notes. Actual results could materially differ from those estimates. Material estimates that are particularly susceptible to significant changes include the allowance for credit losses, accounting for acquired loans, fair value of financial instruments, goodwill and other intangible assets and income taxes.
Loans Held for Sale and Loan Commitments	Loans Held for Sale and Loan Commitments Certain of our residential mortgage loans are originated for sale in the secondary mortgage loan market. Effective January 1, 2017, we made an automatic election to account for all future residential mortgage loans under the fair value option (FVO). The FVO election is intended to better reflect the underlying economics and better facilitate the economic hedging of the loans. The FVO is applied on an instrument by instrument basis and is an irrevocable election. Additionally, with the election of the FVO, fees and costs associated with the origination and acquisition of residential mortgage loans are expensed as incurred, rather than deferred. Changes in fair value under the FVO are recorded in mortgage banking operations non-interest income on the consolidated statements of income. Fair value is determined on the basis of rates obtained in the respective secondary market for the type of loan held for sale. Prior to the FVO election, loans were generally sold at a premium or discount from the carrying amount of the loan which represented the lower of cost or fair value. Gain or loss on the sale of loans is recorded in mortgage banking operations non-interest income. Interest income on loans held for sale is recorded in interest income. We routinely issue interest rate lock commitments for residential mortgage loans that we intend to sell. These interest rate lock commitments are considered derivatives. We also enter into loan sale commitments to sell these loans when funded to mitigate the risk that the market value of residential mortgage loans may decline between the time the rate commitment is issued to the customer and the time we sell the loan. These loan sale commitments are also derivatives. Both types of derivatives are recorded at fair value on the consolidated balance sheets with changes in fair value recorded in mortgage banking operations non-interest income. We also originate loans guaranteed by the Small Business Administration (SBA) for the purchase of businesses, business startups, business expansion, equipment, and working capital. All SBA loans are underwritten and documented as prescribed by the SBA. The portion of SBA loans originated that are guaranteed and intended for sale on the secondary market are classified as held for sale and are carried at the lower of cost or fair value. At the time of the sale, we allocate the carrying value of the entire loan between the guaranteed portion sold and the unguaranteed portion retained based on their relative fair value which results in a discount recorded on the retained portion of the loan. The guaranteed portion is typically sold at a premium and the gain is recognized in other income for any net premium received in excess of the relative fair value of the portion of the loan transferred. The net carrying value of the retained portion of the loans is included in the appropriate loan classification for disclosure purposes, primarily commercial real estate or commercial and industrial.
Premises and Equipment	Premises and Equipment Premises and equipment are stated at cost less accumulated depreciation. Depreciation is computed using the straight-line method over the asset’s estimated useful life. Leasehold improvements are expensed over the lesser of the asset’s estimated useful life or the term of the lease including renewal periods when reasonably assured. Useful lives are dependent upon the nature and condition of the asset and range from 3 to 40 years. Maintenance and repairs are charged to expense as incurred, while major improvements are capitalized and amortized to expense over the identified useful life. Premises and equipment are reviewed for impairment whenever events or changes in circumstances indicate that the carrying value may not be recoverable. Assets to be disposed of are transferred to other assets and are reported at the lower of the carrying amount or fair value less costs to sell.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets Goodwill represents the excess of the cost of an acquisition over the fair value of the net assets acquired. Other intangible assets represent purchased assets that lack physical substance but can be distinguished from goodwill because of contractual or other legal rights. Intangible assets that have finite lives, such as core deposit intangibles, customer relationship intangibles and renewal lists, are amortized over their estimated useful lives and subject to periodic impairment testing. Core deposit intangibles are primarily amortized over ten years using accelerated methods. Customer renewal lists are amortized over their estimated useful lives which range from eight to thirteen years. Goodwill and other intangibles are subject to impairment testing at the reporting unit level, which must be conducted at least annually. We perform impairment testing during the fourth quarter of each year, or more frequently if impairment indicators exist. We also continue to monitor other intangibles for impairment and to evaluate carrying amounts, as necessary. We perform a quantitative assessment to determine whether it is more likely than not that the fair value of a reporting unit is less than its carrying amount. Prior to 2017, if, after assessing updated quantitative factors, we determined it was not more likely than not that the fair value of a reporting unit is less than its carrying amount, we did not have to perform the two-step goodwill impairment test. Determining the fair value of a reporting unit under the first step of the goodwill impairment test and determining the fair value of individual assets and liabilities of a reporting unit under the second step of the goodwill impairment test are judgmental and often involve the use of significant estimates and assumptions. Similarly, estimates and assumptions are used in determining the fair value of other intangible assets. Estimates of fair value are primarily determined using discounted cash flows, market comparisons and recent transactions. These approaches use significant estimates and assumptions including projected future cash flows, discount rates reflecting the market rate of return, projected growth rates and determination and evaluation of appropriate market comparables. Based on the results of quantitative assessments of all reporting units, we concluded that no impairment existed at December 31, 2016. However, future events could cause us to conclude that goodwill or other intangibles have become impaired, which would result in recording an impairment loss. Any resulting impairment loss could have a material adverse impact on our financial condition and results of operations. Beginning in 2017, as permitted under the early adoption provisions of ASU 2017-4, we changed our impairment policy to record an impairment loss, if any, based on the excess of a reporting unit’s carrying amount over its fair value. This change in accounting principle will be applied prospectively. We believe this change in accounting policy is preferable as it reduces the cost and complexity of accounting for goodwill impairment.
Loan Servicing Rights	Loan Servicing Rights We have two primary classes of servicing rights, residential mortgage loan servicing and SBA-guaranteed loan servicing. We recognize the right to service residential mortgage loans and SBA-guaranteed loans for others as an asset whether we purchase the servicing rights or as a result from a sale of loans that we originate when the servicing is contractually separated from the underlying loan and retained by us. We initially record servicing rights at fair value in core deposit and other intangible assets, net on the consolidated balance sheet. Subsequently, servicing rights are measured at the lower of cost or fair value. Servicing rights are amortized in proportion to, and over the period of, estimated net servicing income against servicing income during the period in mortgage banking operations income for residential mortgage loans and other income for SBA-guaranteed loans. The amount and timing of estimated future net cash flows are updated based on actual results and updated projections. Mortgage servicing rights (MSRs) are separated into pools based on common risk characteristics of the underlying loans and evaluated for impairment at least quarterly. SBA-guaranteed servicing rights are evaluated for impairment at least quarterly on an aggregate basis. Impairment, if any, is recognized when carrying value exceeds the fair value as determined by calculating the present value of expected net future cash flows. If impairment exists at the pool level for residential mortgage loans or on an aggregate basis for SBA-guaranteed loans, the servicing right is written down through a valuation allowance and is charged against mortgage banking operations income or other income, respectively.
Bank-Owned Life Insurance (BOLI)	Bank-Owned Life Insurance (BOLI) We have purchased life insurance policies on certain current and former directors, officers and employees for which the Corporation is the owner and beneficiary. These policies are recorded in other assets in the consolidated balance sheet at their cash surrender value, or the amount that could be realized by surrendering the policies. Tax-exempt income from death benefits and changes in the net cash surrender value are recorded in bank owned life insurance income.
Low Income Housing Tax Credit Partnerships	Low Income Housing Tax Credit Partnerships We invest in various affordable housing projects that qualify for low income housing tax credits (LIHTCs). The investments are recorded in other assets on the consolidated balance sheets. These investments generate a return through the realization of federal tax credits. We use the proportional amortization method to account for a majority of our investments in these entities. LIHTCs that do not meet the requirements of the proportional amortization method are recognized using the equity method.
Per Share Amounts	Per Share Amounts Earnings per common share is computed using net income available to common stockholders, which is net income adjusted for preferred stock dividends. Basic earnings per common share is calculated by dividing net income available to common stockholders by the weighted average number of shares of common stock outstanding, net of unvested shares of restricted stock. Diluted earnings per common share is calculated by dividing net income available to common stockholders by the weighted average number of shares of common stock outstanding, adjusted for the dilutive effect of potential common shares issuable for stock options, warrants and restricted shares, as calculated using the treasury stock method. Adjustments to net income available to common stockholders and the weighted average number of shares of common stock outstanding are made only when such adjustments dilute earnings per common share. Beginning in 2017, the assumed proceeds from applying the treasury stock method when computing diluted earnings per share excludes the amount of excess tax benefits that would have been recognized in accumulated paid-in capital in accordance with newly adopted accounting guidance.
Stock Based Compensation	Stock Based Compensation We account for our stock based compensation awards in accordance with ASC 718, Compensation - Stock Compensation, which requires the measurement and recognition of compensation expense, based on estimated fair values, for all stock-based awards, including stock options and restricted stock, made to employees and directors. ASC 718 requires companies to estimate the fair value of stock-based awards on the date of grant. The value of the portion of the award that is ultimately expected to vest is recognized as expense in our consolidated statements of comprehensive income over the shorter of requisite service periods or the period through the date that the employee first becomes eligible to retire. Some of our plans contain performance targets that affect vesting and can be achieved after the requisite service period and are accounted for as performance conditions. Beginning in 2016, the performance target is not reflected in the estimation of the award’s grant date fair value and compensation cost is recognized in the period in which it becomes probable that the performance condition will be achieved. Because stock-based compensation expense is based on awards that are ultimately expected to vest, stock-based compensation expense has been reduced to account for estimated forfeitures. Forfeitures are estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. Beginning in 2017, with the adoption of ASU 2016-9, we elected to change our accounting policy to account for forfeitures as they occur. The estimate for forfeitures prior to adoption of ASU 2016-9 was immaterial to our consolidated financial statements. We believe this change in accounting policy reduces the cost and complexity of accounting for stock-based compensation and is preferable to estimating forfeitures at the time of grant.
New Accounting Standards	NEW ACCOUNTING STANDARDS The following paragraphs summarize accounting pronouncements issued by the Financial Accounting Standards Board (FASB) that we recently adopted or will be adopting in the future. Stock Based Compensation Accounting Standards Update (ASU or Update) 2017-09, Compensation - Stock Compensation: Scope of Modification Accounting, provides guidance about which changes to the terms and conditions of a share-based payment award requires the application of modification accounting. The Update is effective in the first quarter of 2018. Early adoption is permitted. The Update is to be applied prospectively and is not expected to have a material effect on our consolidated financial statements. Securities ASU 2017-08, Receivables-Nonrefundable Fees and Other Costs (Subtopic 310-20): Premium Amortization on Purchased Callable Debt Securities which shortens the amortization period for the premium on certain purchased callable securities to the earliest call date. The accounting for purchased callable debt securities held at a discount does not change. The Update is effective in the first quarter of 2019. Early adoption is permitted. The Update is to be applied using a modified retrospective transition method and is not expected to have a material effect on our consolidated financial statements. Retirement Benefits ASU 2017-07, Compensation—Retirement Benefits: Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost, requires that an employer disaggregate the service cost component from the other components of net benefit cost. The amendments also provide explicit guidance on how to present the service cost component and the other components of net benefit cost in the income statement and allows only the service cost component of net benefit cost to be eligible for capitalization. The Update is effective the first quarter of 2018. Early adoption is permitted. The Update is to be applied using a retrospective transition method to adopt the requirement for separate presentation in the income statement of service costs and other components and a prospective transition method to adopt the requirement to limit the capitalization of benefit costs to the service cost component. We are currently assessing the potential impact to our consolidated financial statements. Goodwill ASU 2017-04, Goodwill and Other (Topic 350): Simplifying the Test for Goodwill Impairment, eliminates the requirement of Step 2 in the current guidance to calculate the implied fair value of goodwill to measure a goodwill impairment charge. Instead, entities will record an impairment charge based on the excess of a reporting unit’s carrying amount over its fair value in Step 1 of the current guidance. The Update is effective the first quarter of 2020. Early adoption is permitted for annual or interim goodwill impairment tests with a measurement date after January 1, 2017. We adopted this Update in 2017 for the next goodwill impairment test. This Update is applied prospectively and is not expected to have a material effect on our consolidated financial statements. Business Combinations ASU 2017-01, Business Combinations (Topic 850): Clarifying the Definition of a Business, clarifies the definition of a business with the objective of providing guidance to assist in the evaluation of whether transactions should be accounted for as acquisitions (disposals) of assets or businesses. The Update is effective for the first quarter of 2018. Early adoption is permitted for transactions that occurred before the issuance date or effective date of the Update if the transactions were not reported in financial statements that have been issued or made available for issuance. We adopted this Update in 2017. This Update was applied prospectively and is not expected to have a material effect on our consolidated financial statements. Statement of Cash Flows ASU 2016-15, Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force), adds or clarifies guidance on eight cash flow issues. The Update is effective the first quarter of 2018. Early adoption is permitted. We are currently assessing the potential impact to our consolidated financial statements. Credit Losses ASU 2016-13, Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments, commonly referred to as “CECL,” replaces the current incurred loss impairment methodology with a methodology that reflects expected credit losses for most financial assets measured at amortized cost and certain other instruments, including loans, held-to-maturity debt securities, net investments in leases and off-balance sheet credit exposures. In addition, the Update will require the use of a modified available-for-sale debt security impairment model and eliminate the current accounting for purchased credit impaired loans and debt securities. The Update is effective the first quarter of 2020. Early adoption is permitted for fiscal years beginning after December 15, 2018. We continue to assess the potential impact to our consolidated financial statements. We are reviewing our business processes, information systems and controls to support recognition and disclosures under this Update. This review includes an assessment of our existing credit models and the financial statement disclosure requirements. The impact of this Update will be dependent on the portfolio composition, credit quality and economic conditions at the time of adoption. Revenue Recognition ASU 2017-05, Other Income-Gains and Losses from the Derecognition of Nonfinancial Assets (Subtopic 610-20): Clarifying the Scope of Asset Derecognition Guidance and Accounting for Partial Sales of Nonfinancial Assets, clarifies the scope for recognizing gains and losses from the transfer of nonfinancial assets in contracts with noncustomers. ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, addresses certain issues in the guidance on assessing collectability, presentation of sales taxes, noncash consideration, and completed contracts and contract modifications at transition. ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, clarifies several aspects of identifying performance obligations and licensing implementation guidance, including guidance that is expected to reduce cost and complexity by eliminating the need to assess whether goods and services are performance obligations if they are immaterial in the context of the contract with the customer. ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), clarifies the guidance on principal versus agent considerations when another party is involved in providing goods and services to a customer. The guidance requires a company to determine whether it is required to provide the specific good or service itself or to arrange for that good or service to be provided by another party. ASU 2014-09, Revenue from Contracts with Customers (Topic 606), modifies the guidance used to recognize revenue from contracts with customers for transfers of goods and services and transfers of nonfinancial assets, unless those contracts are within the scope of other guidance. The guidance also requires new qualitative and quantitative disclosures about contract balances and performance obligations. We expect to adopt ASU 2014-09 in the first quarter of 2018 under the modified retrospective method where the cumulative effect is recognized at the date of initial application. Our evaluation of ASU 2014-09 is ongoing and not complete. The FASB has issued, and may issue in the future, interpretative guidance which may cause our evaluation to change. Based on our evaluation under the current guidance, we estimate that substantially all of our interest income and non-interest income will not be impacted by the adoption of ASU 2014-09 because either the revenue from those contracts with customers is covered by other guidance in U.S. GAAP or the revenue recognition outcomes anticipated with the adoption of ASU 2014-09 will likely be similar to our current revenue recognition practices. We may continue to identify contracts with customers that are out-of-scope or with similar revenue recognition practices through the date of adoption. In addition, we are reviewing our business processes, systems and controls to support recognition and disclosures under the new standard. While we anticipate some changes to revenue recognition within trust, investment management fees and insurance commissions and fees, we have not yet completed our assessment of the potential impact to our consolidated financial statements upon adoption. Stock Based Compensation ASU 2016-09, Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities and classification on the statement of cash flows. The Update was adopted in the first quarter of 2017 by an application method determined by the type of transaction impacted by the adoption. This Update did not have a material effect on our consolidated financial statements. Investments ASU 2016-07, Investments—Equity Method and Joint Ventures (Topic 323): Simplifying the Transition to the Equity Method of Accounting, eliminates the requirement for an investor to retrospectively apply the equity method when an investment that it had accounted for by another method qualifies for use of the equity method. The Update was adopted in the first quarter of 2017 by prospective application. This Update did not have a material effect on our consolidated financial statements. Derivative and Hedging Activities ASU 2016-06, Derivatives and Hedging (Topic 815): Contingent Put and Call Options in Debt Instruments (a consensus of the Emerging Issues Task Force), provides clarification that determination of whether an embedded contingent put or call option in a financial instrument is clearly and closely related to the debt host requires only an analysis of the four-step decision sequence described in ASC 815-15-25-42. The Update was adopted in the first quarter of 2017 by modified retrospective application. This Update did not have a material effect on our consolidated financial statements. ASU 2016-05, Derivatives and Hedging (Topic 815): Effect of Derivative Contract Novations on Existing Hedge Accounting Relationships (a consensus of the Emerging Issues Task Force), clarifies that a change in counterparty to a derivative instrument that has been designated as a hedging instrument under Topic 815 does not, in and of itself, require dedesignation of that hedging relationship provided all other hedge accounting criteria continue to be met. The Update was adopted in the first quarter of 2017 by prospective application. This Update did not have a material effect on our consolidated financial statements. Extinguishments of Liabilities ASU 2016-04, Liabilities—Extinguishments of Liabilities (Subtopic 405-20): Recognition of Breakage for Certain Prepaid Stored-Value Products (a consensus of the Emerging Issues Task Force), requires entities that sell prepaid stored-value products redeemable for goods, services or cash at third-party merchants to recognize breakage. The Update is effective in the first quarter of 2018 with either the modified retrospective method by means of a cumulative-effect adjustment to retained earnings or retrospective application. Early adoption is permitted. This Update is not expected to have a material effect on our consolidated financial statements. Leases ASU 2016-02, Leases (Topic 842), requires lessees to put most leases on their balance sheets but recognize expenses in the income statement similar to current accounting. In addition, the Update changes the guidance for sale-leaseback transactions, initial direct costs and lease executory costs for most entities. All entities will classify leases to determine how to recognize lease related revenue and expense. The Update is effective in the first quarter of 2019 with modified retrospective application including a number of optional practical expedients. Early adoption is permitted. We are currently assessing the potential impact to our consolidated financial statements. Financial Instruments – Recognition and Measurement ASU 2016-01, Financial Instruments—Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities, amends the presentation and accounting for certain financial instruments, including liabilities measured at fair value under the fair value option, and equity investments. The guidance also updates fair value presentation and disclosure requirements for financial instruments measured at amortized cost. The Update is effective in the first quarter of 2018 with a cumulative-effect adjustment as of the beginning of the fiscal year of adoption. Early adoption is prohibited except for the provision requiring the recognition of changes in fair value related to changes in an entity’s own credit risk in other comprehensive income for financial liabilities measured using the fair value option. We are currently assessing the potential impact to our consolidated financial statements.