Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) $ in Thousands	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 824,670	$ 757,231
Tier 1 Common Equity to Risk Weighted Assets	11.26%	11.15%
Tier 1 Common Equity Required to be Well Capitalized	$ 475,876	$ 441,560
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	$ 864,559	$ 791,568
Tier 1 Common Equity to Risk Weighted Assets	11.83%	11.66%
Tier 1 Common Equity Required to be Well Capitalized	$ 475,008	$ 441,378
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	$ 1,071,965	$ 1,010,608
Tier I Capital to Risk Weighted Assets	894,670	827,231
Tier I Capital to Average Assets	$ 894,670	$ 827,231
Total Capital to Risk Weighted Assets, Ratio	0.1464	0.1488
Tier I Capital to Risk Weighted Assets, Ratio	0.1222	0.1218
Tier I Capital to Average Assets, Ratio	0.0973	0.0940
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 1,041,854	$ 974,911
Tier I Capital to Risk Weighted Assets	864,559	791,568
Tier I Capital to Average Assets	$ 864,559	$ 791,568
Total Capital to Risk Weighted Assets, Ratio	0.1426	0.1436
Tier I Capital to Risk Weighted Assets, Ratio	0.1183	0.1166
Tier I Capital to Average Assets, Ratio	0.0943	0.0901
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	$ 768,723	$ 713,289
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	622,300	577,424
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	367,656	352,023
Tier 1 Common Equity Required for Capital Adequacy	$ 512,482	$ 475,526
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	0.1050	0.1050
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	0.0850	0.0850
Tier I Capital to Average Assets, Regulatory Minimum Ratio	0.0400	0.0400
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	$ 767,321	$ 712,995
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	621,165	577,186
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	366,839	351,244
Tier 1 Common Equity Required for Capital Adequacy	$ 511,547	$ 475,330
Total Capital to Risk Weighted Assets, Regulatory Minimum Ratio	0.1050	0.1050
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Ratio	0.0850	0.0850
Tier I Capital to Average Assets, Regulatory Minimum Ratio	0.0400	0.0400
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	$ 585,694	$ 543,458
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.0800	0.0800
Tier One Leverage Capital Required to be Well Capitalized	$ 459,570	$ 440,029
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.0500	0.0500
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 732,118	$ 679,323
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	0.1000	0.1000
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	$ 584,626	$ 543,234
Tier One Risk Based Capital Required to be Well Capitalized to Risk Weighted Assets	0.0800	0.0800
Tier One Leverage Capital Required to be Well Capitalized	$ 458,549	$ 439,055
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.0500	0.0500
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	$ 730,782	$ 679,043
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	0.1000	0.1000