| **Contingent Liabilities**                              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Commitments and Contingencies Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| Contingent Liabilities                                  | Contingent LiabilitiesLegal proceedingsMarket Rate Savings IRA LitigationMcGrogan v. First Commonwealth Bank was filed as a class action on January 12, 2009, in the Court of Common Pleas of Allegheny County, Pennsylvania. The action alleges that the Bank promised class members a minimum interest rate of 8% on its IRA Market Rate Savings Account for as long as the class members kept their money on deposit in the IRA account. The class asserted that the Bank committed fraud, breached its modified contract with the class members, and violated the Pennsylvania Unfair Trade Practice and Consumer Protection Law ("UTPCPL") when it resigned as custodian of the IRA Market Rate Savings Accounts in 2008 and offered the class members a roll-over IRA account with a 3.5% interest rate. Plaintiffs sought monetary damages for the alleged breach of contract, punitive damages for the alleged fraud and Unfair Trade Practice and Consumer Protection Law violations and attorney’s fees.  The court granted class certification as to the breach of modified contract claim and denied class certification as to the fraud and Pennsylvania Unfair Trade Practice and Consumer Protection Law claims. The breach of contract claim was predicated upon a letter sent to customers in 1998 which reversed an earlier decision by the Bank to reduce the rate paid on the accounts. The letter stated, in relevant part, “This letter will serve as notification that a decision has been made to re-establish the rate on your account to eight percent (8)%. This rate will be retroactive to your most recent maturity date and will continue going forward on deposits presently in the account and on annual additions.” On August 30, 2012, the Court entered an order granting the Bank’s motion for summary judgment and dismissed the class action claims. The Court found that the Bank retained the right to resign as custodian of the accounts and that the act of resigning as custodian and closing the accounts did not breach the terms of the underlying IRA contract. On appeal, the Superior Court affirmed the denial of class certification to the claims of fraud in the execution and violation of the UTPCPL.  The Superior Court found that none of the other issues were ripe for appeal.  Jurisdiction was returned to the Court of Common Pleas where the individual fraud and UTPCPL claims of Mr. and Mrs. McGrogan were scheduled for trial.In December 2013, three new complaints were filed by 34 former members of the McGrogan class alleging fraud in the execution and violation of the UTPCPL based upon substantially similar facts to the claims of Mr. and Mrs. McGrogan.In January 2015, the Bank and the various plaintiffs reached a preliminary global settlement of this litigation.  The settlement, which is subject to court approval and other conditions, would resolve the claims of the 36 individual plaintiffs and the claims of the other former members of the McGrogan class.  First Commonwealth established a reserve in the amount of $8.6 million as of December 31, 2014 against the anticipated cost of the settlement.  The contingent reserve is included in "Other liabilities" in the Consolidated Statements of Financial Condition.Other mattersThere are no other material legal proceedings to which First Commonwealth or its subsidiaries are a party, or of which their property is the subject, except proceedings which arise in the normal course of business and, in the opinion of management, will not have a material adverse effect on the consolidated operations or financial position of First Commonwealth or its subsidiaries. |

| X                                                                                                                                                                                                                                                                                               |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
This item represents the entire disclosure related to contingent liabilities.
+ References
No definition available.
+ Details**Name:**fcf_ContingentLiabilitiesTextBlock**Namespace Prefix:**fcf_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                 |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Details**Name:**us-gaap_CommitmentsAndContingenciesDisclosureAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |