Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 896,072	$ 824,670
Tier 1 Common Equity to Risk Weighted Assets	11.09%	11.26%
Tier 1 Common Equity Required to be Well Capitalized	$ 525,131	$ 475,876
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	$ 906,548	$ 864,559
Tier 1 Common Equity to Risk Weighted Assets	11.24%	11.83%
Tier 1 Common Equity Required to be Well Capitalized	$ 524,081	$ 475,008
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	$ 1,160,053	$ 1,071,965
Tier I Capital to Risk Weighted Assets	966,072	894,670
Tier I Capital to Average Assets	$ 966,072	$ 894,670
Total Capital to Risk Weighted Assets, Ratio	0.1436	0.1464
Tier I Capital to Risk Weighted Assets, Ratio	0.1196	0.1222
Tier I Capital to Average Assets, Ratio	0.1018	0.0973
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 1,100,529	$ 1,041,854
Tier I Capital to Risk Weighted Assets	906,548	864,559
Tier I Capital to Average Assets	$ 906,548	$ 864,559
Total Capital to Risk Weighted Assets, Ratio	0.1365	0.1426
Tier I Capital to Risk Weighted Assets, Ratio	0.1124	0.1183
Tier I Capital to Average Assets, Ratio	0.0958	0.0943
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	$ 848,288	$ 768,723
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	686,709	622,300
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	379,527	367,656
Tier 1 Common Equity Required for Capital Adequacy	$ 565,525	$ 512,482
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	$ 846,593	$ 767,321
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	685,337	621,165
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	378,578	366,839
Tier 1 Common Equity Required for Capital Adequacy	$ 564,395	$ 511,547
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	$ 646,314	$ 585,694
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	$ 474,408	$ 459,570
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	$ 807,893	$ 732,118
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	$ 645,023	$ 584,626
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	$ 473,223	$ 458,549
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	$ 806,279	$ 730,782
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	0.1000	0.1000