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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, 2016 | Dec. 31, 2015
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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 | $ 563,262 | $ 556,939
Tier 1 Common Equity to Risk Weighted Assets | 10.06% | 10.00%
Tier 1 Common Equity Required to be Well Capitalized | $ 364,026 | $ 362,091
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 | $ 562,784 | $ 535,814
Tier 1 Common Equity to Risk Weighted Assets | 10.09% | 9.62%
Tier 1 Common Equity Required to be Well Capitalized | $ 362,680 | $ 362,084
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 | $ 687,554 | $ 682,199
Tier I Capital to Risk Weighted Assets | 633,262 | 626,939
Tier I Capital to Average Assets | $ 633,262 | $ 626,939
Total Capital to Risk Weighted Assets, Ratio | 12.28% | 12.25%
Tier I Capital to Risk Weighted Assets, Ratio | 11.31% | 11.25%
Tier I Capital to Average Assets, Ratio | 9.83% | 9.85%
Actual | First Commonwealth Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Total Capital to Risk Weighted Assets | $ 617,076 | $ 656,244
Tier I Capital to Risk Weighted Assets | 562,784 | 600,984
Tier I Capital to Average Assets | $ 562,784 | $ 600,984
Total Capital to Risk Weighted Assets, Ratio | 11.06% | 11.78%
Tier I Capital to Risk Weighted Assets, Ratio | 10.09% | 10.79%
Tier I Capital to Average Assets, Ratio | 8.79% | 9.46%
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 | $ 483,034 | $ 445,650
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 371,026 | 334,237
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 257,776 | 254,631
Tier 1 Common Equity Required for Capital Adequacy | $ 287,020 | $ 250,678
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 8.625% | 8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 6.625% | 6.00%
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 | 5.125% | 4.50%
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 | $ 481,248 | $ 445,642
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 369,654 | 334,231
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 256,214 | 254,095
Tier 1 Common Equity Required for Capital Adequacy | $ 285,959 | $ 250,673
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 8.625% | 8.00%
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio | 6.625% | 6.00%
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 | 5.125% | 4.50%
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 | $ 588,042 | $ 584,915
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 476,034 | 473,503
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 257,776 | 254,631
Tier 1 Common Equity Required for Capital Adequacy | $ 392,028 | $ 389,944
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 | $ 585,867 | $ 584,905
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount | 474,273 | 473,494
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount | 256,214 | 254,095
Tier 1 Common Equity Required for Capital Adequacy | $ 390,578 | $ 389,937
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 | $ 448,032 | $ 445,650
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 | $ 322,220 | $ 318,289
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 | $ 560,040 | $ 557,062
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 | $ 446,375 | $ 445,642
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 | $ 320,268 | $ 317,618
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 | $ 557,969 | $ 557,052
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines | 10.00% | 10.00%

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