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Subordinated Debentures - Additional Information (Detail) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 USD ($) trust Rate | Dec. 31, 2019 USD ($) Rate | May 21, 2018 USD ($)
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount | $ 170,612 | $ 170,450 |
Number of trusts | trust | 2 |  |
First Commonwealth Bank 2028 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount |  |  | $ 50,000
Deferred finance costs | $ 900 |  |
First Commonwealth Bank 2033 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount |  |  | $ 50,000
Deferred finance costs | 1,100 |  |
First Commonwealth Capital Trust III [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount | $ 41,238 | 41,238 |
Redemption price as percentage of principal amount | 100.00% |  |
Deferred finance costs | $ 630 |  |
First Commonwealth Capital Trust II [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Subordinated Debentures outstanding, Amount | $ 30,929 | $ 30,929 |
Redemption price as percentage of principal amount | 100.00% |  |
Deferred finance costs | $ 471 |  |
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Bank 2028 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Debt Instrument, Interest Rate During Period | Rate | 4.88% | 4.88% |
Variable interest rate | 1.845% | 1.845% |
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Bank 2033 [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Debt Instrument, Interest Rate During Period | Rate | 5.50% | 5.50% |
Variable interest rate | 2.37% | 2.37% |
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust III [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 2.85% | 2.85% |
LIBOR [Member] | Subordinated Debt [Member] | First Commonwealth Capital Trust II [Member] |  |  |
Subordinated Borrowing [Line Items] |  |  |
Variable interest rate | 2.85% | 2.85% |

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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 Amount, before accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28555-108399 + 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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