| **New Accounting Pronouncements (Notes)**               | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          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| **Accounting Changes and Error Corrections [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             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| New Accounting Pronouncements                           | New Accounting PronouncementsIn August 2015, the FASB issued ASU No. 2015-14, “Revenue from Contracts with Customers (Topic 606).”  In May 2014, the FASB issued ASU No. 2014-09, "Revenue from Contracts with Customers (Topic 606)," with an original effective date for annual reporting periods beginning after December 15, 2016. The core principle of ASU 2014-09 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. ASU 2015-14 deferred the effective date of ASU 2014-09 to annual periods and interim periods within those annual periods beginning after December 15, 2017. We are currently evaluating the potential impact of ASU 2015-14 on our financial statements.In January 2016, the FASB issued ASU No. 2016-01, "Financial Instruments - Overall (Subtopic 825-10)," which addresses certain aspects of recognition, measurement, presentation and disclosure of financial instruments.  This ASU addresses: 1. requiring equity investments to be measured at fair value, recognizing the changes in fair value through net income; 2. simplifying the impairment assessment of equity investments without readily determinable fair values by requiring a qualitative assessment to identify impairment; 3. eliminating the requirement to disclose the fair value of financial instruments measured at amortized cost for entities that are not public entities; 4. eliminating the requirement for public entities to disclose the methods and significant assumptions used to estimate the fair value of financial instruments measured at amortized cost; 5. requiring public entities to use the exit price notion when measuring the fair value of financial instruments; 6. requiring 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 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. requiring separate presentation of financial assets and financial liabilities by measurement category and form of financial asset on the balance sheet or in the accompanying notes to the financial statements; and 8. clarifying 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. This ASU is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017 for public entities and for fiscal years beginning after December 15, 2018 for entities that are not public entities.  The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations.In February 2016, the FASB issued ASU No. 2016-02, "Leases (Topic 842)," in order to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements.  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. This ASU is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018.  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. Entities have the option to use certain relief; full retrospective application is prohibited. We are currently evaluating the potential impact of  ASU 2016-02 on our financial statements. In March 2016, FASB issued ASU No. 2016-05, "Derivatives and Hedging (Topic 815)," which clarifies that a change in the counterparty to a derivative instrument that has been designated as the hedging instrument under Topic 815 does not, in and of itself, require de-designation of that hedging relationship provided that all other hedge accounting criteria continue to be met.  This ASU is effective for public entities beginning after December 15, 2016, and interim periods within those fiscal years.  The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations.In March 2016, FASB issued ASU No. 2016-09, “Compensation - Stock Compensation (Topic 718) - Improvements to Employee Share-Based Payment Accounting.”  This update requires all income tax effects of awards to be recognized in the income statement when the awards vest or are settled. It also allows an employer to repurchase more of an employee’s shares than it can today for tax withholding purposes without triggering liability accounting and to make a policy election for forfeitures as they occur. The guidance is effective for public business entities for fiscal years beginning after December 15, 2016, and interim periods within those years. Early adoption is permitted. The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations.In June 2016, the FASB issued ASU No. 2016-13, “Financial Instruments – Credit Losses (Topic 326), Measurement of Credit Losses on Financial Instruments,” which amends the guidance for recognizing credit losses from an “incurred loss” methodology that delays recognition of credit losses until it is probable a loss has been incurred to an expected credit loss methodology. The guidance requires the use of the modified retrospective transition method by means of a cumulative-effect adjustment to equity as of the beginning of the period in which the guidance is adopted. The standard is effective for the Company as of January 1, 2020. Management is currently evaluating the impact of the amended guidance on First Commonwealth’s financial condition or results of operationsIn August 2016, the FASB issued ASU No. 2016-15, “Statement of Cash Flows (Topic 230),” which provides guidance on eight specific cash flow issues: 1. debt prepayment or extinguishment costs; 2. settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates; 3. contingent consideration payments made after a business combination; 4. proceeds from the settlement of insurance claims; 5. proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; 6. distributions received from equity method investees; 7. beneficial interests in securitizations transactions; and 8. separately identifiable cash flows and application of the predominance principle.  This ASU provides additional guidance for these eight issues, reducing current and potential diversity in practice.  This standard is effective for the Company as of January 1, 2018. Management is currently evaluating the impact of the amended guidance on First Commonwealth’s financial condition or results of operations. |

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| - Definition
Disclosure of accounting policy pertaining to new accounting pronouncements that may impact the entity's financial reporting. Includes, but is not limited to, quantification of the expected or actual impact.
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