| **Investment Securities**                                                                                 | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             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| **Investment Securities [Abstract]**                                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  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| Investments in Debt and Marketable Equity Securities (and Certain Trading Assets) Disclosure [Text Block] | Investment SecuritiesSecurities Available for SaleBelow is an analysis of the amortized cost and estimated fair values of securities available for sale at: September 30, 2016 December 31, 2015 AmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFair Value AmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFair Value (dollars in thousands)Obligations of U.S. Government Agencies:               Mortgage-Backed Securities – Residential$16,059 $1,917 $— $17,976 $20,034 $2,071 $(13) $22,092Obligations of U.S. Government-Sponsored Enterprises:             Mortgage-Backed Securities – Residential692,092 13,090 (556) 704,626 778,476 7,983 (8,882) 777,577Mortgage-Backed Securities – Commercial1 — — 1 28 — — 28Other Government-Sponsored Enterprises19,300 7 — 19,307 19,201 2 (85) 19,118Obligations of States and Political Subdivisions27,073 878 — 27,951 27,066 532 — 27,598Corporate Securities5,901 618 — 6,519 1,897 422 — 2,319Pooled Trust Preferred Collateralized Debt Obligations43,020 677 (8,088) 35,609 42,239 916 (7,497) 35,658Total Debt Securities803,446 17,187 (8,644) 811,989 888,941 11,926 (16,477) 884,390Equities1,670 — — 1,670 2,170 — — 2,170Total Securities Available for Sale$805,116 $17,187 $(8,644) $813,659 $891,111 $11,926 $(16,477) $886,560Mortgage backed securities include mortgage backed obligations of U.S. Government agencies and obligations of U.S. Government-sponsored enterprises. These obligations have contractual maturities ranging from less than one year to approximately 30 years with lower anticipated lives to maturity due to prepayments. All mortgage backed securities contain a certain amount of risk related to the uncertainty of prepayments of the underlying mortgages. Interest rate changes have a direct impact upon prepayment speeds; therefore, First Commonwealth uses computer simulation models to test the average life and yield volatility of all mortgage backed securities under various interest rate scenarios to monitor the potential impact on earnings and interest rate risk positions.Expected maturities will differ from contractual maturities because issuers may have the right to call or repay obligations with or without call or prepayment penalties. Other fixed income securities within the portfolio also contain prepayment risk.The amortized cost and estimated fair value of debt securities available for sale at September 30, 2016, by contractual maturity, are shown below.  AmortizedCost EstimatedFair Value (dollars in thousands)Due within 1 year$5,600 $5,602Due after 1 but within 5 years17,697 17,756Due after 5 but within 10 years27,073 27,951Due after 10 years44,924 38,077 95,294 89,386Mortgage-Backed Securities (a)708,152 722,603Total Debt Securities$803,446 $811,989 (a)Mortgage Backed Securities include an amortized cost of $16.1 million and a fair value of $18.0 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $692.1 million and a fair value of $704.6 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac. Proceeds from sales, gross gains (losses) realized on sales, maturities and other-than-temporary impairment charges related to securities available for sale were as follows for the nine months endedSeptember 30: 2016 2015 (dollars in thousands)Proceeds from sales$55,744 $—Gross gains (losses) realized:   Sales Transactions:   Gross gains$304 $—Gross losses(276) — 28 —Maturities and impairment   Gross gains— 125Gross losses— —Other-than-temporary impairment— — — 125Net gains and impairment$28 $125Securities available for sale with an estimated fair value of $498.5 million and $416.1 million were pledged as of September 30, 2016 and December 31, 2015, respectively, to secure public deposits and for other purposes required or permitted by law.Securities Held to MaturityBelow is an analysis of the amortized cost and fair values of debt securities held to maturity at: September 30, 2016 December 31, 2015 AmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFair Value AmortizedCost GrossUnrealizedGains GrossUnrealizedLosses EstimatedFair Value (dollars in thousands)Obligations of U.S. Government Agencies:               Mortgage-Backed Securities – Residential$4,615 $154 $— $4,769 $4,775 $— $(7) $4,768Mortgage-Backed Securities- Commercial35,625 225 — 35,850 16,843 — (247) 16,596Obligations of U.S. Government-Sponsored Enterprises:               Mortgage-Backed Securities – Residential297,681 5,865 — 303,546 315,609 30 (1,824) 313,815Mortgage-Backed Securities – Commercial14,809 378 — 15,187 15,187 — (178) 15,009Obligations of States and Political Subdivisions36,783 884 (25) 37,642 31,910 301 (58) 32,153Total Securities Held to Maturity$389,513 $7,506 $(25) $396,994 $384,324 $331 $(2,314) $382,341The amortized cost and estimated fair value of debt securities held to maturity at September 30, 2016, by contractual maturity, are shown below.  Expected maturities will differ from contractual maturities because borrowers may have the right to call or repay obligations with or without call or prepayment penalties. AmortizedCost EstimatedFair Value (dollars in thousands)Due within 1 year$— $—Due after 1 but within 5 years1,223 1,249Due after 5 but within 10 years29,368 30,125Due after 10 years6,192 6,268 36,783 37,642Mortgage-Backed Securities (a)352,730 359,352Total Debt Securities$389,513 $396,994(a)Mortgage Backed Securities include an amortized cost of $40.2 million and a fair value of $40.6 million for Obligations of U.S. Government agencies issued by Ginnie Mae and an amortized cost of $312.5 million and a fair value of $318.7 million for Obligations of U.S. Government-sponsored enterprises issued by Fannie Mae and Freddie Mac.Securities held to maturity with an amortized cost of $281.9 million and $45.7 million were pledged as of September 30, 2016 and December 31, 2015, respectively, to secure public deposits and for other purposes required or permitted by law. |

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| - Definition
Investment Securities [Abstract]
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No definition available.
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| - Definition
The entire disclosure for  investments in certain debt and equity securities.
+ References
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Debt Security -URI http://asc.fasb.org/extlink&oid=6509901Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6379141&loc=d3e15032-111544Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -Glossary Equity Security -URI http://asc.fasb.org/extlink&oid=6511694Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563Reference 8: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27357-111563Reference 9: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27405-111563
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