| **New Accounting Pronouncements (Notes)**               | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          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| **Accounting Changes and Error Corrections [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
| 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, theFASB issued ASU No. 2014-09, "Revenue from Contracts with Customers (Topic 606)", with an original effective date forannual reporting periods beginning after December 15, 2016. The core principle of ASU 2014-09 is that an entity shouldrecognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the considerationto which the entity expects to be entitled in exchange for those goods or services. ASU 2015-14 deferred the effective date ofASU 2014-09 to annual periods and interim periods within those annual periods beginning after December 15, 2017. A significant component of the Company’s revenues, net interest income on financial assets and liabilities, is excluded from the scope of the amended guidance. The Company is the process of identifying and evaluating certain noninterest revenue streams, including trust and asset management fees, deposit related fees, interchange fees, and merchant income to determine the potential impact of the guidance on the Company’s Consolidated Financial Statements. The Company has decided to use the modified retrospective method for transition in which the cumulative effect will be recognized at the date of adoption with no restatement of comparative periods presented.  The Company expects additional financial statement disclosures and associated internal controls to be implemented along with the adoption of this ASU.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 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 operations.In 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. The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations.In January 2017, the FASB issued ASU No. 2017-01, "Business Combinations (Topic 805), Clarifying the Definition of a Business" which provides a screen to determine when a set of assets and activities (a "set") is not a business.  The screen requires, that when substantially all of the fair value of gross assets acquired (or disposed of) is concentrated in a single identifiable asset or a group of similar identifiable assets, the set is not a business.  This screen thereby reduces the number of transactions that need to be further evaluated.  If the screen is not met, this ASU 1. requires that to be considered a business, a set must include, at a minimum, an input and substantive process that significantly contributes to the ability to create output and 2. removes the evaluation of whether a market participant could replace the missing elements.  The amendment provides a framework to assist entities in evaluating whether both an input and substantive process is present.  The framework includes two sets of criteria to consider that depend on whether a set has outputs. This ASU also narrows the definition of the term output so that the term is consistent with how outputs are described in Topic 606.  This standard is effective for interim and annual periods for fiscal years beginning after December 15, 2017.  The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations.In January 2017, the FASB issued ASU No. 2017-04, "Intangibles-Goodwill and Other (Topic 350), Simplifying the Test for Goodwill Impairment" which simplifies the subsequent measurement of goodwill by eliminating Step 2 from the goodwill impairment test.  Under this ASU, an entity should perform its annual, or interim, goodwill impairment test by comparing the fair value of a reporting unit with its carrying amount.  Impairment should be recognized for the amount by which the carrying amount exceeds the reporting unit's fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to the reporting unit.  Income tax effects from any tax deductible goodwill should be taken into consideration of the carrying amount of the reporting unit when measuring for goodwill impairment, if applicable.  An entity still has the option to perform the qualitative assessment for the reporting unit to determine if the quantitative impairment test is necessary.  This standard is effective for interim and annual periods for fiscal years beginning after December 15, 2019.  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 2017, the FASB issued ASU No. 2017-07, "Compensation-Retirement Benefits (Topic 715), Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost" which shortens the amortization period for certain callable debt securities held at a premium. Specifically, the ASU requires the premium to be amortized to the earliest call date. The amendments do not require an accounting change for securities held at a discount; the discount continues to be amortized to maturity.  This standard is effective for annual periods for fiscal years beginning after December 15, 2019.  The adoption of this ASU is not expected to have a material impact on First Commonwealth’s financial condition or results of operations. |

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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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