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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, 2018 | Dec. 31, 2017
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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 | $ 697,881 | $ 623,252
Tier 1 Common Equity to Risk Weighted Assets | 11.13% | 10.32%
Tier 1 Common Equity Required to be Well Capitalized | $ 407,557 | $ 392,620
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 | $ 734,246 | $ 658,861
Tier 1 Common Equity to Risk Weighted Assets | 11.73% | 10.94%
Tier 1 Common Equity Required to be Well Capitalized | $ 406,925 | $ 391,315
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 | $ 918,786 | $ 745,473
Tier I Capital to Risk Weighted Assets | 767,881 | 691,993
Tier I Capital to Average Assets | $ 767,881 | $ 691,993
Total Capital to Risk Weighted Assets, Ratio | 14.65% | 12.34%
Tier I Capital to Risk Weighted Assets, Ratio | 12.25% | 11.46%
Tier I Capital to Average Assets, Ratio | 10.28% | 9.74%
Actual | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets | $ 885,151 | $ 712,341
Tier I Capital to Risk Weighted Assets | 734,246 | 658,861
Tier I Capital to Average Assets | $ 734,246 | $ 658,861
Total Capital to Risk Weighted Assets, Ratio | 14.14% | 11.83%
Tier I Capital to Risk Weighted Assets, Ratio | 11.73% | 10.94%
Tier I Capital to Average Assets, Ratio | 9.84% | 9.30%
Minimum Capital Required - Basel III Phase-In Schedule | First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 619,173 | $ 558,728
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 493,771 | 437,922
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 298,856 | 284,100
Tier 1 Common Equity Required for Capital Adequacy | $ 399,719 | $ 347,317
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 9.875% | 9.25%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 7.875% | 7.25%
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 | 6.375% | 5.75%
Minimum Capital Required - Basel III Phase-In Schedule | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 618,213 | $ 556,872
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 493,005 | 436,467
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 298,340 | 283,344
Tier 1 Common Equity Required for Capital Adequacy | $ 399,100 | $ 346,163
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 9.875% | 9.25%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 7.875% | 7.25%
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 | 6.375% | 5.75%
Minimum Capital Required - Basel III Fully Phased-In | First Commonwealth Financial Corporation |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 658,361 | $ 634,232
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 532,959 | 513,426
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 298,856 | 284,100
Tier 1 Common Equity Required for Capital Adequacy | $ 438,907 | $ 422,821
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 - Basel III Fully Phased-In | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | $ 657,341 | $ 632,124
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 532,133 | 511,720
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 298,340 | 283,344
Tier 1 Common Equity Required for Capital Adequacy | $ 438,227 | $ 421,416
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 | $ 501,608 | $ 483,224
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 | $ 373,570 | $ 355,125
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 | $ 627,011 | $ 604,030
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 | $ 500,831 | $ 481,619
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 | $ 372,926 | $ 354,180
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 | $ 626,039 | $ 602,023
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | 10.00% | 10.00%

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