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Subordinated Debentures - Subordinated Debentures Outstanding (Detail) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2019 | Dec. 31, 2018 | May 21, 2018
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount | $ 170,450 | $ 170,288 |
First Commonwealth Bank due 2028 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due | Jun. 01, 2028 |  |
Subordinated Debentures outstanding, Amount | $ 49,222 | 49,131 |
First Commonwealth Bank due 2033 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due | Jun. 01, 2033 |  |
Subordinated Debentures outstanding, Amount | $ 49,061 | 48,990 |
First Commonwealth Capital Trust II [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due | Jan. 23, 2034 |  |
Subordinated Debentures outstanding, Amount | $ 30,929 | 30,929 |
First Commonwealth Capital Trust III [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Due | Apr. 06, 2034 |  |
Subordinated Debentures outstanding, Amount | $ 41,238 | $ 41,238 |
First Commonwealth Bank 2028 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount |  |  | $ 50,000
First Commonwealth Bank 2033 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount |  |  | $ 50,000
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust II [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 2.85% | 2.85% |
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Capital Trust III [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 2.85% | 2.85% |
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2028 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 1.845% | 1.845% |
Debt Instrument, Interest Rate During Period | 4.88% | 4.88% |
Subordinated Debt [Member] | LIBOR [Member] | First Commonwealth Bank 2033 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 2.37% | 2.37% |
Debt Instrument, Interest Rate During Period | 5.50% | 5.50% |

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- Definition The average effective interest rate during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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