| **Impairment of Investment Securities**                       | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           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| **Impairment of Investment Securities Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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| Impairment of Investment Securities                           | Impairment of Investment SecuritiesSecurities Available for Sale and Held to MaturityAs required by FASB ASC Topic 320, “Investments – Debt and Equity Securities,” credit-related other-than-temporary impairment on debt securities is recognized in earnings, while non-credit related other-than-temporary impairment on debt securities not expected to be sold is recognized in OCI. During the nine months endedSeptember 30, 2016 and 2015, no other-than-temporary impairment charges were recognized. First Commonwealth utilizes the specific identification method to determine the net gain or loss on debt securities and the average cost method to determine the net gain or loss on equity securities.We review our investment portfolio on a quarterly basis for indications of impairment. This review includes analyzing the length of time and the extent to which the fair value has been lower than the cost, the financial condition and near-term prospects of the issuer, including any specific events which may influence the operations of the issuer and whether we are more likely than not to sell, or be required to sell, the security. We evaluate whether we are more likely than not to sell debt securities based upon our investment strategy for the particular type of security, our cash flow needs, liquidity position, capital adequacy, tax position and interest rate risk position. In addition, the risk of future other-than-temporary impairment may be influenced by additional bank failures, weakness in the U.S. economy, changes in real estate values and additional interest deferrals in our pooled trust preferred collateralized debt obligations. Our pooled trust preferred collateralized debt obligations are beneficial interests in securitized financial assets within the scope of FASB ASC Topic 325, “Investments – Other,” and are therefore evaluated for other-than-temporary impairment using management’s best estimate of future cash flows. If these estimated cash flows indicate that it is probable that an adverse change in cash flows has occurred, then other-than-temporary impairment would be recognized in accordance with FASB ASC Topic 320. There is a risk that First Commonwealth will record other-than-temporary impairment charges in the future. See Note 10, “Fair Values of Assets and Liabilities,” for additional information.The following table presents the gross unrealized losses and estimated fair values at September 30, 2016 for both available for sale and held to maturity securities by investment category and time frame for which securities have been in a continuous unrealized loss position:  Less Than 12 Months 12 Months or More Total EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses (dollars in thousands)Obligations of U.S. Government-Sponsored Enterprises:           Mortgage-Backed Securities – Residential$59,691 $(137) $60,909 $(419) $120,600 $(556)Obligations of States and Political Subdivisions1,781 (25) — — 1,781 (25)Pooled Trust Preferred Collateralized Debt Obligations— — 29,896 (8,088) 29,896 (8,088)Total Securities$61,472 $(162) $90,805 $(8,507) $152,277 $(8,669)At September 30, 2016, fixed income securities issued by U.S. Government-sponsored enterprises comprised 6% of total unrealized losses due to changes in market interest rates. Pooled trust preferred collateralized debt obligations accounted for 93% of the unrealized losses primarily due to the illiquid market for this investment type.  At September 30, 2016, there are 22 debt securities in an unrealized loss position.The following table presents the gross unrealized losses and estimated fair values at December 31, 2015 by investment category and time frame for which securities have been in a continuous unrealized loss position: Less Than 12 Months 12 Months or More Total EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses EstimatedFair Value GrossUnrealizedLosses (dollars in thousands)Obligations of U.S. Government Agencies:           Mortgage-Backed Securities – Residential$6,798 $(20) $— $— $6,798 $(20)Mortgage-Backed Securities - Commercial16,596 (247) — — 16,596 (247)Obligations of U.S. Government-Sponsored Enterprises:           Mortgage-Backed Securities – Residential436,011 (3,293) 263,119 (7,413) 699,130 (10,706)Mortgage-Backed Securities – Commercial15,009 (178) — — 15,009 (178)Other Government-Sponsored Enterprises12,316 (85) — — 12,316 (85)Obligation of States and Political Subdivisions7,208 (58) — — 7,208 (58)Pooled Trust Preferred Collateralized Debt Obligations— — 29,957 (7,497) 29,957 (7,497)Total Securities$493,938 $(3,881) $293,076 $(14,910) $787,014 $(18,791)As of September 30, 2016, our corporate securities had an amortized cost and an estimated fair value of $5.9 million and $6.5 million, respectively.  As of December 31, 2015, our corporate securities had an amortized cost and estimated fair value of $1.9 million and $2.3 million, respectively.  Corporate securities are comprised of debt for large regional banks.  There were no corporate securities in an unrealized loss position as of September 30, 2016 and December 31, 2015.  When unrealized losses exist on these investments, management reviews each of the issuer’s asset quality, earnings trends and capital position, to determine whether issues in an unrealized loss position were other-than-temporarily impaired. All interest payments on the corporate securities are being made as contractually required.As of September 30, 2016, the book value of our pooled trust preferred collateralized debt obligations totaled $43.0 million with an estimated fair value of $35.6 million, which includes securities comprised of 268 banks and other financial institutions. All of our pooled securities are mezzanine tranches, three of which have no senior class remaining in the issue. The credit ratings on all of our issues are below investment grade.  At the time of initial issue, the subordinated tranches ranged in size from approximately 7% to 35% of the total principal amount of the respective securities and no more than 5% of any pooled security consisted of a security issued by any one institution. As of September 30, 2016, after taking into account management’s best estimates of future interest deferrals and defaults, three of our securities had no excess subordination in the tranches we own and six of our securities had excess subordination which ranged from 2% to 82% of the current performing collateral. The following table provides information related to our pooled trust preferred collateralized debt obligations as of September 30, 2016:DealClass BookValue Estimated FairValue UnrealizedGain(Loss) Moody’s/FitchRatings NumberofBanks DeferralsandDefaultsas a % ofCurrentCollateral ExcessSubordinationas a % ofCurrentPerformingCollateral(dollars in thousands)Pre TSL IVMezzanine $1,830 $1,331 $(499) B1/BB 6 18.05% 60.61%Pre TSL VIIMezzanine 3,119 3,513 394 Ca/- 14 47.77 0.00Pre TSL VIIIMezzanine 2,063 2,025 (38) C/C 28 44.37 0.00Pre TSL IXMezzanine 2,403 1,893 (510) B1/C 37 27.83 12.79Pre TSL XMezzanine 1,707 1,887 180 Caa1/C 42 31.58 2.17Pre TSL XIIMezzanine 5,839 4,584 (1,255) B3/C 64 24.50 0.00Pre TSL XIIIMezzanine 12,888 10,504 (2,384) Ba3/C 54 11.75 48.41Pre TSL XIVMezzanine 12,960 9,559 (3,401) B1/CC 54 13.45 37.92MMCap IMezzanine 211 313 102 Ca/C 8 58.11 81.65Total  $43,020 $35,609 $(7,411)        Lack of liquidity in the market for trust preferred collateralized debt obligations, below investment grade credit ratings and market uncertainties related to the financial industry are factors contributing to the impairment on these securities.In October 2016, the Company received notice that the Senior note holders of Pre TSL VII elected to liquidate all assets of the trust. The sale of the assets and early redemption of the security is anticipated to be completed in the fourth quarter of 2016. Estimated proceeds are expected to be in line with our book value.All of the Company's pooled trust preferred securities are included in the non-exclusive list issued by the regulatory agencies and therefore are not considered covered funds under the Volcker Rule.On a quarterly basis we evaluate our debt securities for other-than-temporary impairment. During the three and nine months endedSeptember 30, 2016 and 2015, there were no credit-related other-than-temporary impairment charges recognized on our pooled trust preferred collateralized debt obligations. When evaluating these investments, we determine a credit-related portion and a non-credit related portion of other-than-temporary impairment. The credit-related portion is recognized in earnings and represents the difference between book value and the present value of future cash flows. The non-credit related portion is recognized in OCI and represents the difference between the fair value of the security and the amount of credit-related impairment. A discounted cash flow analysis provides the best estimate of credit-related other-than-temporary impairment for these securities.Additional information related to the discounted cash flow analysis follows:Our pooled trust preferred collateralized debt obligations are measured for other-than-temporary impairment within the scope of FASB ASC Topic 325 by determining whether it is probable that an adverse change in estimated cash flows has occurred. Determining whether there has been an adverse change in estimated cash flows from the cash flows previously projected involves comparing the present value of remaining cash flows previously projected against the present value of the cash flows estimated at September 30, 2016. We consider the discounted cash flow analysis to be our primary evidence when determining whether credit related other-than-temporary impairment exists. Results of a discounted cash flow test are significantly affected by other variables, such as the estimate of future cash flows, credit worthiness of the underlying banks and determination of probability of default of the underlying collateral. The following provides additional information for each of these variables:•Estimate of Future Cash Flows – Cash flows are constructed in an INTEX cash flow model which includes each deal’s structural features. Projected cash flows include prepayment assumptions, which are dependent on the issuer's asset size and coupon rate.  For collateral issued by financial institutions over $15 billion in asset size with a coupon over 7%, a 100% prepayment rate is assumed. Financial institutions over $15 billion with a coupon of 7% or under are assigned a prepayment rate of 40% for two years and 2% thereafter. Financial institutions with assets between $2 billion and $15 billion with coupons over 7% are assigned a 5% prepayment rate. For financial institutions below $2 billion, if the coupon is over 10%, a prepayment rate of 5% is assumed and for all other issuers, there is no prepayment assumption incorporated into the cash flows.  The modeled cash flows are then used to estimate if all the scheduled principal and interest payments of our investments will be returned. •Credit Analysis – A quarterly credit evaluation is performed for each of the 268 banks comprising the collateral across the various pooled trust preferred securities. Our credit evaluation considers all evidence available to us and includes the nature of the issuer’s business, its years of operating history, corporate structure, loan composition, loan concentrations, deposit mix, asset growth rates, geographic footprint and local economic environment. Our analysis focuses on profitability, return on assets, shareholders’ equity, net interest margin, credit quality ratios, operating efficiency, capital adequacy and liquidity.•Probability of Default – A probability of default is determined for each bank and is used to calculate the expected impact of future deferrals and defaults on our expected cash flows. Each bank in the collateral pool is assigned a probability of default for each year until maturity. Currently, any bank that is in default is assigned a 100% probability of default and a 0% projected recovery rate. All other banks in the pool are assigned a probability of default based on their unique credit characteristics and market indicators with a 10% projected recovery rate. For the majority of banks currently in deferral we assume the bank continues to defer and will eventually default and, therefore, a 100% probability of default is assigned. However, for some deferring collateral there is the possibility that they will become current on interest or principal payments at some point in the future and in those cases a probability that the deferral will ultimately cure is assigned. The probability of default is updated quarterly. As of September 30, 2016, default probabilities for performing collateral ranged from 0.33% to 75%.Our credit evaluation provides a basis for determining deferral and default probabilities for each underlying piece of collateral. Using the results of the credit evaluation, the next step of the process is to look at pricing of senior debt or credit default swaps for the issuer (or where such information is unavailable, for companies having similar credit profiles as the issuer). The pricing of these market indicators provides the information necessary to determine appropriate default probabilities for each bank.In addition to the above factors, our evaluation of impairment also includes a stress test analysis which provides an estimate of excess subordination for each tranche. We stress the cash flows of each pool by increasing current default assumptions to the level of defaults that results in an adverse change in estimated cash flows. This stressed breakpoint is then used to calculate excess subordination levels for each pooled trust preferred security. The results of the stress test allow management to identify those pools that are at a greater risk for a future break in cash flows so that we can monitor banks in those pools more closely for potential deterioration of credit quality.Our cash flow analysis as of September 30, 2016, indicates that no credit-related other-than-temporary impairment has occurred on our pooled trust preferred securities during the nine months endedSeptember 30, 2016. Based upon the analysis performed by management, it is probable that three of our pooled trust preferred securities will experience principal and interest shortfalls and therefore appropriate other-than-temporary charges were recorded in prior periods. These securities are identified in the table on page 17 with 0.00% “Excess Subordination as a Percentage of Current Performing Collateral.” For the remaining securities listed in that table, our analysis as of September 30, 2016 indicates it is probable that we will collect all contractual principal and interest payments.  For four of those securities, PreTSL IX, PreTSL XIII, PreTSL XIV and MMCap I, other-than-temporary impairment charges were recorded in prior periods; however, due to improvement in the expected cash flows of these securities, it is now probable that all contractual payments will be received.During 2008, 2009 and 2010, other-than-temporary impairment charges were recognized on all of our pooled trust preferred securities, except for PreTSL IV. Our cash flow analysis as of September 30, 2016, for all of these impaired securities indicates that it is now probable we will collect principal and interest in excess of what was estimated at the time other-than-temporary impairment charges were recorded. This change can be attributed to improvement in the underlying collateral for these securities and has resulted in the present value of estimated future principal and interest payments exceeding the securities' current book value. The excess for each bond of the present value of future cash flows over our current book value ranges from 19% to 123% and will be recognized as an adjustment to yield over the remaining life of these securities.  The excess subordination recognized as an adjustment to yield is reflected in the following table as increases in cash flows expected to be collected.  The following table provides a cumulative roll forward of credit losses recognized in earnings for debt securities held and not intended to be sold: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2016 2015 2016 2015 (dollars in thousands)Balance, beginning (a)$24,310 $25,366 $24,851 $26,246Credit losses on debt securities for which other-than-temporary impairment was not previously recognized— — — —Additional credit losses on debt securities for which other-than-temporary impairment was previously recognized— — — —Increases in cash flows expected to be collected, recognized over the remaining life of the security (b)(270) (255) (811) (917)Reduction for debt securities called during the period— — — (218)Balance, ending$24,040 $25,111 $24,040 $25,111 (a)The beginning balance represents credit related losses included in other-than-temporary impairment charges recognized on debt securities in prior periods.(b)Represents the increase in cash flows recognized in interest income during the period.In the first nine months of 2016 and 2015, no other-than-temporary impairment charges were recorded on equity securities. On a quarterly basis, management evaluates equity securities for other-than-temporary impairment by reviewing the severity and duration of decline in estimated fair value, research reports, analysts’ recommendations, credit rating changes, news stories, annual reports, regulatory filings, impact of interest rate changes and other relevant information. As of September 30, 2016 and 2015, there were no equity securities in an unrealized loss position.Other InvestmentsAs a member of the Federal Home Loan Bank ("FHLB"), First Commonwealth is required to purchase and hold stock in the FHLB to satisfy membership and borrowing requirements. The level of stock required to be held is dependent on the amount of First Commonwealth's mortgage-related assets and outstanding borrowings with the FHLB. This stock is restricted in that it can only be sold to the FHLB or to another member institution, and all sales of FHLB stock must be at par. As a result of these restrictions, FHLB stock is unlike other investment securities insofar as there is no trading market for FHLB stock and the transfer price is determined by FHLB membership rules and not by market participants. As of September 30, 2016 and December 31, 2015, our FHLB stock totaled $54.1 million and $63.0 million, respectively, and is included in “Other investments” on the Condensed Consolidated Statements of Financial Condition.FHLB stock is held as a long-term investment and its value is determined based on the ultimate recoverability of the par value. First Commonwealth evaluates impairment quarterly and has concluded that the par value of its investment in FHLB stock will be recovered.  Accordingly, no impairment charge was recorded on these securities during the three and nine months ended September 30, 2016. |

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This item represents the entire disclosure related to the impairment of our debt and equity securities.
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