Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) $ in Thousands	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 1,052,814	$ 896,072
Tier 1 Common Equity to Risk Weighted Assets	11.16%	11.09%
Tier 1 Common Equity Required to be Well Capitalized	$ 613,172	$ 525,131
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	$ 1,030,975	$ 906,548
Tier 1 Common Equity to Risk Weighted Assets	10.97%	11.24%
Tier 1 Common Equity Required to be Well Capitalized	$ 610,727	$ 524,081
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	$ 1,314,021	$ 1,160,053
Tier I Capital to Risk Weighted Assets	1,122,814	966,072
Tier I Capital to Average Assets	$ 1,122,814	$ 966,072
Total Capital to Risk Weighted Assets, Ratio	0.1393	0.1436
Tier I Capital to Risk Weighted Assets, Ratio	0.1190	0.1196
Tier I Capital to Average Assets, Ratio	0.1004	0.1018
Actual \| First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 1,222,182	$ 1,100,529
Tier I Capital to Risk Weighted Assets	1,030,975	906,548
Tier I Capital to Average Assets	$ 1,030,975	$ 906,548
Total Capital to Risk Weighted Assets, Ratio	0.1301	0.1365
Tier I Capital to Risk Weighted Assets, Ratio	0.1097	0.1124
Tier I Capital to Average Assets, Ratio	0.0924	0.0958
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	$ 990,508	$ 848,288
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	801,840	686,709
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	447,542	379,527
Tier 1 Common Equity Required for Capital Adequacy	$ 660,339	$ 565,525
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	$ 986,558	$ 846,593
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	798,642	685,337
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	446,530	378,578
Tier 1 Common Equity Required for Capital Adequacy	$ 657,705	$ 564,395
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	$ 754,673	$ 646,314
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	$ 559,427	$ 474,408
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	$ 943,341	$ 807,893
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	$ 751,663	$ 645,023
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	$ 558,163	$ 473,223
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	$ 939,579	$ 806,279
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	0.1000	0.1000

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Amount of total capital as defined by regulatory framework.

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Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework.

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