XML 138 R123.htm IDEA: XBRL DOCUMENT
v3.8.0.1

Retirement Plans - Schedule of Weighted-Average Assumptions Used to Determine Benefit Obligation (Detail) - Benefit Obligation [Member] | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Net Periodic Benefit Cost Assumptions [Line Items] |  |  |
Discount rate | 3.37% | 3.74% | 3.88%
Health care cost trend: Initial | 6.00% | 6.00% | 6.25%
Health care cost trend: Ultimate | 4.75% | 4.75% | 4.75%
Year ultimate reached | 2023 | 2022 | 2022

X
- Definition Defined Benefit Plan Initial Health Care Cost Trend Rate + References No definition available. + Details
Name: | fcf_DefinedBenefitPlanInitialHealthCareCostTrendRate
Namespace Prefix: | fcf_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Net Periodic Benefit Cost Assumptions [Line Items] + References No definition available. + Details
Name: | fcf_NetPeriodicBenefitCostAssumptionsLineItems
Namespace Prefix: | fcf_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (k)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Ultimate trend rate for health care cost for defined benefit postretirement plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanUltimateHealthCareCostTrendRate1
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Year ultimate health care cost trend rate is expected to be reached, in CCYY format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (l) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanYearHealthCareCostTrendRateReachesUltimateTrendRate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RetirementPlanFundingStatusAxis=fcf_BenefitObligationMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |