| **Fair Values of Assets and Liabilities (Tables)**                                | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|-----------------------------------------------------------------------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
||
| **Fair Value Disclosures [Abstract]**                                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Fair Value Inputs, Assets, Quantitative Information                               | In accordance with ASU 2011-4, the following table provides information related to quantitative inputs and assumptions used in Level 3 fair value measurements. Fair Value (dollarsin thousands) ValuationTechnique Unobservable Inputs Range /(weighted average)Pooled Trust Preferred Securities$35,779 Discounted Cash Flow Probability of default 0% - 100% (13.72%)     Prepayment rates 0% - 73.52% (5.09%)     Discount rates 5.25% - 12.00% (a)Equities1,920 Par Value N/A N/AImpaired Loans2,903 (b) Reserve study Discount rate 10.00%     Gas per MCF $2.71 - $3.48 (c)     Oil per BBL/d $55.00 (c) 520 (b) Discounted Cash Flow Discount Rate 1.90%Other Real Estate Owned8 Internal Valuation N/A N/A (a)incorporates spread over risk free rate related primarily to credit quality and illiquidity of securities.(b)the remainder of impaired loans valued using Level 3 inputs are not included in this disclosure as the values of those loans are based on bankruptcy agreement documentation.(c)unobservable inputs are defined as follows: MCF - million cubic feet; BBL/d - barrels per day.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis      | The tables below present the balances of assets and liabilities measured at fair value on a recurring basis: September 30, 2015 Level 1 Level 2 Level 3 Total (dollars in thousands)Obligations of U.S. Government Agencies:       Mortgage-Backed Securities - Residential$— $23,281 $— $23,281Obligations of U.S. Government-Sponsored Enterprises:       Mortgage-Backed Securities - Residential— 824,152 — 824,152Mortgage-Backed Securities - Commercial— 38 — 38Other Government-Sponsored Enterprises— 135,781 — 135,781Obligations of States and Political Subdivisions— 27,449 — 27,449Corporate Securities— 2,333 — 2,333Pooled Trust Preferred Collateralized Debt Obligations— — 35,779 35,779Total Debt Securities— 1,013,034 35,779 1,048,813Equities— — 1,920 1,920Total Securities Available for Sale— 1,013,034 37,699 1,050,733Other Investments— 53,976 — 53,976Loans held for sale— 4,986 — 4,986Other Assets(a)— 16,303 — 16,303Total Assets$— $1,088,299 $37,699 $1,125,998Other Liabilities(a)$— $14,333 $— $14,333Total Liabilities$— $14,333 $— $14,333(a)Hedging and non-hedging interest rate derivatives December 31, 2014 Level 1 Level 2 Level 3 Total (dollars in thousands)Obligations of U.S. Government Agencies:       Mortgage-Backed Securities - Residential$— $25,936 $— $25,936Obligations of U.S. Government-Sponsored Enterprises:       Mortgage-Backed Securities - Residential— 950,881 — 950,881Mortgage-Backed Securities - Commercial— 74 — 74Other Government-Sponsored Enterprises— 267,877 — 267,877Obligations of States and Political Subdivisions— 27,377 — 27,377Corporate Securities— 7,255 — 7,255Pooled Trust Preferred Collateralized Debt Obligations— — 28,999 28,999Total Debt Securities— 1,279,400 28,999 1,308,399Equities— — 1,420 1,420Total Securities Available for Sale— 1,279,400 30,419 1,309,819Other Investments— 44,545 — 44,545Loans Held for Sale— 2,502 — 2,502Other Assets(a)— 11,186 — 11,186Total Assets$— $1,337,633 $30,419 $1,368,052Other Liabilities(a)$— $10,671 $— $10,671Total Liabilities$— $10,671 $— $10,671(a)Hedging and non-hedging interest rate derivatives                                                                                                                                                             |
| Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation | For the nine months endedSeptember 30, changes in Level 3 assets and liabilities measured at fair value on a recurring basis are summarized as follows: 2015 Pooled Trust Preferred Collateralized Debt Obligations Equities Total (dollars in thousands)Balance, beginning of period$28,999 $1,420 $30,419Total gains or losses     Included in earnings105 — 105Included in other comprehensive income7,729 — 7,729Purchases, issuances, sales, and settlements     Purchases— 500 500Issuances— — —Sales— — —Settlements(1,054) — (1,054)Transfers into Level 3— — —Balance, end of period$35,779 $1,920 $37,699  2014 Pooled Trust Preferred Collateralized Debt Obligations Equities Loans Held for Sale Total (dollars in thousands)Balance, beginning of period$23,523 $1,420 $— $24,943Total gains or losses       Included in earnings— — 77 77Included in other comprehensive income7,266 — — 7,266Purchases, issuances, sales, and settlements       Purchases— — — —Issuances— — — —Sales— — (3,112) (3,112)Settlements(1,333) — — (1,333)Transfers into Level 3— — 3,035 3,035Balance, end of period$29,456 $1,420 $— $30,876 2015 Pooled Trust Preferred Collateralized Debt Obligations Equities Total (dollars in thousands)Balance, beginning of period$35,521 $1,920 $37,441Total gains or losses     Included in earnings— — —Included in other comprehensive income258 — 258Purchases, issuances, sales, and settlements     Purchases— — —Issuances— — —Sales— — —Settlements— — —Transfers into Level 3— — —Balance, end of period$35,779 $1,920 $37,699 2014 Pooled Trust Preferred Collateralized Debt Obligations Equities Loans Held for Sale Total (dollars in thousands)Balance, beginning of period$28,154 $1,420 $— 29,574Total gains or losses       Included in earnings— — — —Included in other comprehensive income1,470 — — 1,470Purchases, issuances, sales, and settlements       Purchases— — — —Issuances— — — —Sales— — — —Settlements(168) — — (168)Transfers into Level 3— — — —Balance, end of period$29,456 $1,420 $— $30,876 |
| Schedule of Assets Measured on Non-Recurring Basis                                | The tables below present the balances of assets measured at fair value on a non-recurring basis at: September 30, 2015 Level 1 Level 2 Level 3 Total (dollars in thousands)Impaired loans$— $26,916 $9,656 $36,572Other real estate owned— 12,386 8 12,394Total Assets$— $39,302 $9,664 $48,966 December 31, 2014 Level 1 Level 2 Level 3 Total (dollars in thousands)Impaired loans$— $34,864 $10,926 $45,790Other real estate owned— 7,828 153 7,981Total Assets$— $42,692 $11,079 $53,771                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
| Losses Realized on Assets Measured on Non-Recurring Basis                         | The following losses were realized on the assets measured on a nonrecurring basis: For the Three Months Ended September 30, For the Nine Months Ended September 30, 2015 2014 2015 2014 (dollars in thousands)Impaired loans$(2,838) $456 $(3,532) $(1,291)Other real estate owned(78) (51) (1,205) (1,109)Total losses$(2,916) $405 $(4,737) $(2,400)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
| Carrying Amounts and Fair Values of Financial Instruments                         | The following table presents carrying amounts and fair values of First Commonwealth’s financial instruments: September 30, 2015   Fair Value Measurements Using: CarryingAmount Total Level 1 Level 2 Level 3 (dollars in thousands)Financial assets         Cash and due from banks$69,235 $69,235 $69,235 $— $—Interest-bearing deposits3,529 3,529 3,529 — —Securities available for sale1,050,733 1,050,733 — 1,013,034 37,699Securities held to maturity154,035 154,837 — 154,837 —Other investments53,976 53,976 — 53,976 —Loans held for sale4,986 4,986 — 4,986 —Loans4,575,735 4,596,889 — 26,916 4,569,973Financial liabilities         Deposits4,161,490 4,164,660 — 4,164,660 —Short-term borrowings1,329,794 1,329,920 — 1,329,920 —Subordinated debt72,167 62,227 — — 62,227Long-term debt39,052 39,704 — 39,704 — December 31, 2014   Fair Value Measurements Using: CarryingAmount Total Level 1 Level 2 Level 3 (dollars in thousands)Financial assets         Cash and due from banks$72,276 $72,276 $72,276 $— $—Interest-bearing deposits2,262 2,262 2,262 — —Securities available for sale1,309,819 1,309,819 — 1,279,400 30,419Other investments44,545 44,545 — 44,545 —Loans held for sale2,502 2,502 — 2,502 —Loans4,457,308 4,439,766 — 34,864 4,404,902Financial liabilities         Deposits4,315,511 4,319,997 — 4,319,997 —Short-term borrowings1,105,876 1,105,867 — 1,105,867 —Subordinated debt72,167 62,815 — — 62,815Long-term debt89,459 90,319 — 90,319 —                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |

| X                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Losses realized on assets measured on non-recurring basis.
+ References
No definition available.
+ Details**Name:**fcf_LossesRealizedOnAssetsMeasuredOnNonrecurringBasisTableTextBlock**Namespace Prefix:**fcf_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of assets and liabilities by class, including financial instruments measured at fair value that are classified in shareholders' equity, if any, that are measured at fair value on a nonrecurring basis in periods after initial recognition (for example, impaired assets). Disclosures may include, but are not limited to: (a) the fair value measurements recorded and the reasons for the measurements and (b) the level within the fair value hierarchy in which the fair value measurements are categorized in their entirety (levels 1, 2, 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19296-110258
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnNonrecurringBasisTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the fair value measurement of assets using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes during the period attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets) and gains or losses recognized in other comprehensive income (loss), and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs), by class of asset.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19279-110258
+ Details**Name:**us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13476-108611Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13467-108611Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6447952&loc=d3e13220-108610Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 30 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14172-108612Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=49121117&loc=d3e13433-108611
+ Details**Name:**us-gaap_FairValueByBalanceSheetGroupingTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
+ Details**Name:**us-gaap_FairValueDisclosuresAbstract**Namespace Prefix:**us-gaap_**Data Type:**xbrli:stringItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of quantitative information about the inputs used in the fair value measurement of assets. This disclosure may include, but is not limited to, the fair value of the asset, valuation technique used to measure fair value, the inputs used to measure fair value, the ranges of the inputs, and the weighted averages of the inputs.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258
+ Details**Name:**us-gaap_FairValueInputsAssetsQuantitativeInformationTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |
| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19190-110258
+ Details**Name:**us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock**Namespace Prefix:**us-gaap_**Data Type:**nonnum:textBlockItemType**Balance Type:**na**Period Type:**duration |