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
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%