Basis of Presentation	9 Months Ended
Sep. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accounting and reporting policies of First Commonwealth Financial Corporation and its subsidiaries (“First Commonwealth” or the “Company”) conform with generally accepted accounting principles in the United States of America (“GAAP”). The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Actual realized amounts could differ from those estimates. In the opinion of management, the unaudited interim condensed consolidated financial statements include all adjustments (consisting of only normal recurring adjustments) necessary for a fair presentation of First Commonwealth’s financial position, results of operations, comprehensive income, cash flows and changes in shareholders’ equity as of and for the periods presented. The results of operations for the nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the full year of 2018. These interim financial statements should be read in conjunction with First Commonwealth’s 2017 Annual Report on Form 10-K. Adoption of New Accounting Standards On January 1, 2018, First Commonwealth adopted ASU 2014-09, "Revenue from Contracts with Customers" ("ASC 606") and all subsequent amendments to the ASU, which creates a single framework for recognizing revenue from contracts with customers that fall within its scope and revises when it is appropriate to recognize a gain(loss) from the transfer of nonfinancial assets, such as OREO. The majority of the Company's revenues come from interest income and other sources, including loans and securities, that are outside the scope of ASC 606. The Company's services that fall within the scope of ASC 606 are presented within non-interest income and are recognized as revenue as the Company satisfies its obligation to the customer. Services within the scope of ASC 606 include trust income, service charges on deposits, insurance and retail brokerage commissions, interchange fees and gain(loss) on other real estate owned ("OREO"). Refer to Note 15, "Revenue Recognition" for further discussion on the Company's accounting policies for revenue sources within the scope of ASC 606. The Company adopted ASC 606 using the modified retrospective method applied to all contracts not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under ASC 606 while prior period amounts continue to be reported in accordance with legacy GAAP. The adoption of ASC 606 did not result in a significant change to the accounting for any of the in-scope revenue streams; as such, no cumulative effect adjustment was recorded. On January 1, 2018, First Commonwealth elected to adopt ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220)." As part of this adoption, First Commonwealth has elected to reclassify the income tax effects resulting from tax reform from accumulated other comprehensive income to retained earnings on a portfolio basis. ASU 2018-02 provides for the reclassification of the stranded tax effects resulting from the Tax Cuts and Jobs Act. As of January 1, 2018, First Commonwealth reclassified $1.3 million from accumulated other comprehensive income to retained earnings in relation to the stranded tax effect which included accumulated other comprehensive income recognized on available-for-sale investment securities, interest rate swaps and other post-retirement benefits. This reclassification is shown as an adjustment to the beginning of the year balances and can be seen in the Condensed Consolidated Statements of Changes in Shareholders' Equity. In January 2016, the FASB issued ASU No. 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities.” This ASU addresses certain aspects of recognition, measurement, presentation, and disclosure of financial instruments by making targeted improvements to GAAP as follows: (1) require equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) 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, and when that assessment indicates that impairment exists, requiring the entity to measure the investment at fair value; (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 (that is, securities or loans and receivables) 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 adoption of ASU No. 2016-01 on January 1, 2018 did not have a material impact on the Company’s Consolidated Financial Statements. In accordance with this ASU, and as reflected in Note 11, "Fair Values of Assets and Liabilities," the Company measured the fair value of its loan portfolio as of September 30, 2018 using an exit price notion. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold and interest-bearing bank deposits. Generally, federal funds are sold for one-day periods.

Basis of Presentation

The accounting and reporting policies of First Commonwealth Financial Corporation and its subsidiaries (“First Commonwealth” or the “Company”) conform with generally accepted accounting principles in the United States of America (“GAAP”). The preparation of financial statements in conformity with GAAP requires management to make estimates, assumptions and judgments that affect the amounts reported in the financial statements and accompanying notes. Actual realized amounts could differ from those estimates. In the opinion of management, the unaudited interim condensed consolidated financial statements include all adjustments (consisting of only normal recurring adjustments) necessary for a fair presentation of First Commonwealth’s financial position, results of operations, comprehensive income, cash flows and changes in shareholders’ equity as of and for the periods presented.

The results of operations for the nine months ended September 30, 2018 are not necessarily indicative of the results that may be expected for the full year of 2018. These interim financial statements should be read in conjunction with First Commonwealth’s 2017 Annual Report on Form 10-K.

Adoption of New Accounting Standards

On January 1, 2018, First Commonwealth adopted ASU 2014-09, "Revenue from Contracts with Customers" ("ASC 606") and all subsequent amendments to the ASU, which creates a single framework for recognizing revenue from contracts with customers that fall within its scope and revises when it is appropriate to recognize a gain(loss) from the transfer of nonfinancial assets, such as OREO. The majority of the Company's revenues come from interest income and other sources, including loans and securities, that are outside the scope of ASC 606. The Company's services that fall within the scope of ASC 606 are presented within non-interest income and are recognized as revenue as the Company satisfies its obligation to the customer. Services within the scope of ASC 606 include trust income, service charges on deposits, insurance and retail brokerage commissions, interchange fees and gain(loss) on other real estate owned ("OREO"). Refer to Note 15, "Revenue Recognition" for further discussion on the Company's accounting policies for revenue sources within the scope of ASC 606. The Company adopted ASC 606 using the modified retrospective method applied to all contracts not completed as of January 1, 2018. Results for reporting periods beginning after January 1, 2018 are presented under ASC 606 while prior period amounts continue to be reported in accordance with legacy GAAP. The adoption of ASC 606 did not result in a significant change to the accounting for any of the in-scope revenue streams; as such, no cumulative effect adjustment was recorded.

On January 1, 2018, First Commonwealth elected to adopt ASU 2018-02, "Income Statement - Reporting Comprehensive Income (Topic 220)." As part of this adoption, First Commonwealth has elected to reclassify the income tax effects resulting from tax reform from accumulated other comprehensive income to retained earnings on a portfolio basis. ASU 2018-02 provides for the reclassification of the stranded tax effects resulting from the Tax Cuts and Jobs Act. As of January 1, 2018, First Commonwealth reclassified $1.3 million from accumulated other comprehensive income to retained earnings in relation to the stranded tax effect which included accumulated other comprehensive income recognized on available-for-sale investment securities, interest rate swaps and other post-retirement benefits. This reclassification is shown as an adjustment to the beginning of the year balances and can be seen in the Condensed Consolidated Statements of Changes in Shareholders' Equity.

In January 2016, the FASB issued ASU No. 2016-01, “Recognition and Measurement of Financial Assets and Financial Liabilities.” This ASU addresses certain aspects of recognition, measurement, presentation, and disclosure of financial instruments by making targeted improvements to GAAP as follows: (1) require equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) 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, and when that assessment indicates that impairment exists, requiring the entity to measure the investment at fair value; (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 (that is, securities or loans and receivables) 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 adoption of ASU No. 2016-01 on January 1, 2018 did not have a material impact on the Company’s Consolidated Financial Statements. In accordance with this ASU, and as reflected in Note 11, "Fair Values of Assets and Liabilities," the Company measured the fair value of its loan portfolio as of September 30, 2018 using an exit price notion.

For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold and interest-bearing bank deposits. Generally, federal funds are sold for one-day periods.