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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, 2017 | Dec. 31, 2016
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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 | $ 623,252 | $ 563,262
Tier 1 Common Equity to Risk Weighted Assets | 10.32% | 10.06%
Tier 1 Common Equity Required to be Well Capitalized | $ 392,620 | $ 364,026
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 | $ 658,861 | $ 562,784
Tier 1 Common Equity to Risk Weighted Assets | 10.94% | 10.09%
Tier 1 Common Equity Required to be Well Capitalized | $ 391,315 | $ 362,680
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 | $ 745,473 | $ 687,554
Tier I Capital to Risk Weighted Assets | 691,993 | 633,262
Tier I Capital to Average Assets | $ 691,993 | $ 633,262
Total Capital to Risk Weighted Assets, Ratio | 12.34% | 12.28%
Tier I Capital to Risk Weighted Assets, Ratio | 11.46% | 11.31%
Tier I Capital to Average Assets, Ratio | 9.74% | 9.83%
Actual | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets | $ 712,341 | $ 617,076
Tier I Capital to Risk Weighted Assets | 658,861 | 562,784
Tier I Capital to Average Assets | $ 658,861 | $ 562,784
Total Capital to Risk Weighted Assets, Ratio | 11.83% | 11.06%
Tier I Capital to Risk Weighted Assets, Ratio | 10.94% | 10.09%
Tier I Capital to Average Assets, Ratio | 9.30% | 8.79%
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 | $ 558,728 | $ 483,034
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 437,922 | 371,026
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 284,100 | 257,776
Tier 1 Common Equity Required for Capital Adequacy | $ 347,317 | $ 287,020
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 9.25% | 8.625%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 7.25% | 6.625%
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 | 5.75% | 5.125%
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 | $ 556,872 | $ 481,248
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 436,467 | 369,654
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 283,344 | 256,214
Tier 1 Common Equity Required for Capital Adequacy | $ 346,163 | $ 285,959
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 9.25% | 8.625%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 7.25% | 6.625%
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 | 5.75% | 5.125%
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 | $ 634,232 | $ 588,042
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 513,426 | 476,034
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 284,100 | 257,776
Tier 1 Common Equity Required for Capital Adequacy | $ 422,821 | $ 392,028
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 | $ 632,124 | $ 585,867
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 511,720 | 474,273
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 283,344 | 256,214
Tier 1 Common Equity Required for Capital Adequacy | $ 421,416 | $ 390,578
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 | $ 483,224 | $ 448,032
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 | $ 355,125 | $ 322,220
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 | $ 604,030 | $ 560,040
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 | $ 481,619 | $ 446,375
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 | $ 354,180 | $ 320,268
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 | $ 602,023 | $ 557,969
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | 10.00% | 10.00%

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