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Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
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First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 Common Equity to Risk Weighted Assets | $ 757,231 | $ 730,526
Tier 1 Common Equity to Risk Weighted Assets | 11.15% | 10.91%
Tier 1 Common Equity Required to be Well Capitalized | $ 441,560 | $ 435,420
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets | 6.50% | 6.50%
First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier 1 Common Equity to Risk Weighted Assets | $ 791,568 | $ 759,398
Tier 1 Common Equity to Risk Weighted Assets | 11.66% | 11.36%
Tier 1 Common Equity Required to be Well Capitalized | $ 441,378 | $ 434,575
Tier 1 Common Equity Required to be Well Capitalized to Risk Weighted Assets | 6.50% | 6.50%
Actual | First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets | $ 1,010,608 | $ 954,991
Tier I Capital to Risk Weighted Assets | 827,231 | 800,526
Tier I Capital to Average Assets | $ 827,231 | $ 800,526
Total Capital to Risk Weighted Assets, Ratio | 14.88% | 14.26%
Tier I Capital to Risk Weighted Assets, Ratio | 12.18% | 11.95%
Tier I Capital to Average Assets, Ratio | 9.40% | 10.17%
Actual | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets | $ 974,911 | $ 913,863
Tier I Capital to Risk Weighted Assets | 791,568 | 759,398
Tier I Capital to Average Assets | $ 791,568 | $ 759,398
Total Capital to Risk Weighted Assets, Ratio | 14.36% | 13.67%
Tier I Capital to Risk Weighted Assets, Ratio | 11.66% | 11.36%
Tier I Capital to Average Assets, Ratio | 9.01% | 9.66%
Minimum Capital Required | First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 713,289 | $ 703,370
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 577,424 | 569,395
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 352,023 | 314,963
Tier 1 Common Equity Required for Capital Adequacy | $ 475,526 | $ 468,914
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 10.50% | 10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 8.50% | 8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio | 4.00% | 4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets | 7.00% | 7.00%
Minimum Capital Required | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 712,995 | $ 702,006
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 577,186 | 568,291
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 351,244 | 314,338
Tier 1 Common Equity Required for Capital Adequacy | $ 475,330 | $ 468,004
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 10.50% | 10.50%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 8.50% | 8.50%
Tier I Capital to Average Assets, Regulatory Minimum Ratio | 4.00% | 4.00%
Tier 1 Common Equity Required for Capital Adequacy to Risk Weighted Assets | 7.00% | 7.00%
Required to be Considered Well Capitalized | First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier One Risk Based Capital Required to be Well Capitalized | $ 543,458 | $ 535,901
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets | 8.00% | 8.00%
Tier One Leverage Capital Required to be Well Capitalized | $ 440,029 | $ 393,704
Tier One Leverage Capital Required to be Well Capitalized to Average Assets | 5.00% | 5.00%
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | $ 679,323 | $ 669,877
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | 10.00% | 10.00%
Required to be Considered Well Capitalized | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Tier One Risk Based Capital Required to be Well Capitalized | $ 543,234 | $ 534,862
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets | 8.00% | 8.00%
Tier One Leverage Capital Required to be Well Capitalized | $ 439,055 | $ 392,922
Tier One Leverage Capital Required to be Well Capitalized to Average Assets | 5.00% | 5.00%
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | $ 679,043 | $ 668,577
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | 10.00% | 10.00%

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