| **Basis of Presentation**                                                           | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
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| **Organization, Consolidation and Presentation of Financial Statements [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
| Basis of Presentation                                                               | Basis of PresentationThe 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 endedSeptember 30, 2016 are not necessarily indicative of the results that may be expected for the full year of 2016. These interim financial statements should be read in conjunction with First Commonwealth’s 2015 Annual Report on Form 10-K. 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. |

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| - Definition
The entire disclosure for the business description and basis of presentation concepts.  Business description describes the nature and type of organization including but not limited to organizational structure as may be applicable to holding companies, parent and subsidiary relationships, business divisions, business units, business segments, affiliates and information about significant ownership of the reporting entity.  Basis of presentation describes the underlying basis used to prepare the financial statements (for example, US Generally Accepted Accounting Principles, Other Comprehensive Basis of Accounting, IFRS).
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