Regulatory Restrictions and Capital Adequacy - Schedule of Capital Amount and Ratio Considered to be Capitalized (Detail) $ in Thousands	Dec. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
First Commonwealth Financial Corporation
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 Common Equity to Risk Weighted Assets	$ 1,224,913	$ 1,137,183
Tier 1 Common Equity to Risk Weighted Assets	12.05%	12.13%
Tier 1 Common Equity Required to be Well Capitalized	$ 660,859	$ 609,324
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	$ 1,186,263	$ 1,108,623
Tier 1 Common Equity to Risk Weighted Assets	11.69%	11.85%
Tier 1 Common Equity Required to be Well Capitalized	$ 659,473	$ 608,039
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	$ 1,471,506	$ 1,366,963
Tier I Capital to Risk Weighted Assets	1,294,913	1,207,183
Tier I Capital to Average Assets	$ 1,294,913	$ 1,207,183
Total Capital to Risk Weighted Assets, Ratio	0.1447	0.1458
Tier I Capital to Risk Weighted Assets, Ratio	0.1274	0.1288
Tier I Capital to Average Assets, Ratio	0.1086	0.1063
Actual | First Commonwealth Bank
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk Weighted Assets	$ 1,362,592	$ 1,268,403
Tier I Capital to Risk Weighted Assets	1,186,263	1,108,623
Tier I Capital to Average Assets	$ 1,186,263	$ 1,108,623
Total Capital to Risk Weighted Assets, Ratio	0.1343	0.1356
Tier I Capital to Risk Weighted Assets, Ratio	0.1169	0.1185
Tier I Capital to Average Assets, Ratio	0.0997	0.0978
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	$ 1,067,542	$ 984,292
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	864,201	796,808
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	476,821	454,457
Tier 1 Common Equity Required for Capital Adequacy	$ 711,695	$ 656,195
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	$ 1,065,303	$ 982,216
Tier I Capital to Risk Weighted Assets, Regulatory Minimum Capital Amount	862,388	795,128
Tier I Capital to Average Assets, Regulatory Minimum Capital Amount	475,776	453,467
Tier 1 Common Equity Required for Capital Adequacy	$ 710,202	$ 654,811
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	$ 813,365	$ 749,937
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	$ 596,026	$ 568,071
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	$ 1,016,707	$ 937,421
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	$ 811,659	$ 748,355
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	$ 594,720	$ 566,834
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	$ 1,014,574	$ 935,444
Total Capital to Risk Weighted Assets, Well Capitalized Regulatory Guidelines	0.1000	0.1000