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
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%