| **Supplemental Cash Flow Disclosures - Non-cash Investing and Financing Activities (Detail) (USD $)In Thousands, unless otherwise specified** | 12 Months Ended                                                                                      |
|-----------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------|
||
| **Cash paid during the year for:**                                                                                                            |                                                                                                 |   |   |
| Interest                                                                                                                                      | $ 18,943us-gaap_InterestPaid                                                                         | $ 23,022us-gaap_InterestPaid | $ 31,597us-gaap_InterestPaid |
| Income taxes                                                                                                                                  | 10,700us-gaap_IncomeTaxesPaidNet                                                                     | 3,080us-gaap_IncomeTaxesPaidNet | 11,641us-gaap_IncomeTaxesPaidNet |
| **Non-cash investing and financing activities:**                                                                                              |                                                                                                 |   |   |
| ESOP loan reductions                                                                                                                          | 0us-gaap_EmployeeStockOwnershipPlanESOPDebtStructureDirectLoanEmployerCashPaymentsUsedForDebtService | 0us-gaap_EmployeeStockOwnershipPlanESOPDebtStructureDirectLoanEmployerCashPaymentsUsedForDebtService | 1,600us-gaap_EmployeeStockOwnershipPlanESOPDebtStructureDirectLoanEmployerCashPaymentsUsedForDebtService |
| Loans transferred to other real estate owned and repossessed assets                                                                           | 5,061us-gaap_TransferOfOtherRealEstate                                                               | 12,326us-gaap_TransferOfOtherRealEstate | 4,979us-gaap_TransferOfOtherRealEstate |
| Other Real Estate Owned and Settled Out of Period                                                                                             | 0fcf_TransferOfOtherRealEstateOwned                                                                  | 348fcf_TransferOfOtherRealEstateOwned | 0fcf_TransferOfOtherRealEstateOwned |
| Fair value of loans transferred from held to maturity to available for sale                                                                   | 3,035us-gaap_TransferOfPortfolioLoansAndLeasesToHeldForSale1                                         | 20,135us-gaap_TransferOfPortfolioLoansAndLeasesToHeldForSale1 | 0us-gaap_TransferOfPortfolioLoansAndLeasesToHeldForSale1 |
| Gross (decrease) increase in market value adjustment to securities available for sale                                                         | 24,601fcf_GrossIncreaseInMarketValueAdjustmentToSecuritiesAvailableForSale                           | (33,792)fcf_GrossIncreaseInMarketValueAdjustmentToSecuritiesAvailableForSale | (874)fcf_GrossIncreaseInMarketValueAdjustmentToSecuritiesAvailableForSale |
| Unsettled treasury stock repurchases                                                                                                          | $ 0fcf_Unsettledtreasurystockrepurchases                                                             | $ 0fcf_Unsettledtreasurystockrepurchases | $ 1,222fcf_Unsettledtreasurystockrepurchases |

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Gross increase in market value adjustment to securities available for sale.
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No definition available.
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| X                                                                                                                                                                                                                                                                                                                    |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of other real estate transferred from the entity's loan portfolio in noncash transactions.
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| X                                                                                                                                                                                                                                                          |
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| - Definition
Unsettled treasury stock repurchases
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash provided by the employer through contributions or dividends that the ESOP used in the period to repay debt and interest on a third-party loan made directly to the ESOP.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes.
+ References
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| - Definition
The amount of cash paid for interest during the period.
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|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value transferred out of real estate owned (REO) in noncash transactions.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Value of loans and leases held in the entity's portfolio transferred to loans and leases held-for-sale in noncash transactions.
+ References
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